If you fly to Phoenix, you can visit Frank Lloyd Wright's Taliesin West and then drive the 4 hours to Puerto Penasco via the Lukeville Border Crossing (not busy). Its best to visit El Pinacate from the North. At the 52 km mark on the HWY is a 76 km NON-paved loop road (dirt) that leads to some of the reserves massive craters (3 in total). The dirt road is maintained and passable to cars (though rough), but it does take at least 3-4 hours with stops. The visitor center is accessed at the 72 km mark on the HWY and is well-displayed by large signs. You won't miss it. The turn-off is a paved 7 km road that ends with one of the most scenic areas of the reserve.

For more details see my double review of this WHS

I would suggest a 3-4 day trip that begins with Phoenix to Puerto Penasco and then Puerto Penasco back to Phoenix. After returning to Phoenix fly to Tijuana and then to Loreto. I am seeing Volaris flights as low as $300. Phoenix - Tijuana - Loreto (one-way). From Loreto its about a 5 hour drive to El Vizcaíno Biosphere Reserve. Don't try to do El Pinacate all in one day. Best visit the craters on the first day and when returning North to the US check out the visitor center/lava fields/sand dunes.

The thing is, I am doing this as part of a Grand Mexico Trip. So I am coming from Mexico City. The US would be a huge detour and I'd need a visa.

Tijuana does connect with Loreto indeed. But to get back to the Mexican "mainland", I'd need a different departure city.
As far as I have researched now, I am even considering doing it in 2 segments: (1) Mex City - Mexicali and then the El Pinacate WHS; then a few weeks traveling around Mexico, and when I end up in Mexico City again, (2) Mex City - La Paz, rent a car there and drive to Loreto (for Gulf of California WHS) and San Ignacio (for El Vizcaino WHS and Sierra de Francisco Rock Paintings WHS). But it is all very cumbersome and costly.

You can visit Isla San Jorge (or Bird Island) from Puerto Penasco, which is part of the Gulf of California WHS. Tours are easily booked. The maps included here show other sites that one might not immediately think of (several places are included that are not in Baja California or Baja California Sur). You could focus on Mexico (except Baja California) and only miss 3 WHS in the whole country. Based on your previous trips, I understand you like to go big! Good luck and I am excited to see what your trip actually entails.

Whale Sanctuary of El Vizcaino (1993)
Rock Paintings of the Sierra de San Francisco (1993)
Archipiélago de Revillagigedo (2016)

Thanks for the encouragement winterkjm! I will post my full itinerary when I have it all figured out, the plan is to leave January 10. Unfortunately, I will be missing Archipiélago de Revillagigedo as well (forever, I guess, unless I take up diving).

Regarding
: I was a bit doubtful whether it is in the core zone. It depends on how you interpret the map: there are islands colored in green (which definitely are the focus areas) and there are the borders of the much larger nature reserves (where Bird Island falls into). See https://www.researchgate.net/figure/Upper-Gulf-of-California-and-Colorado-River-Delta-Biosphere-Reserve-including-the-core_fig1_301600285. But from looking at the size of this component (Upper Gulf of & Colorado River Delta Biosphere Reserve) as stated in the IUCN file - 454,591 ha of marine area - it seems that all is included. Also, it seems logical that if you want to protect the vaquitas, you need a large marine component and not just islands. In that case, the zone just extends to the coast west of Puerto Penasco and will include Bird Island.

Bahia Loreto may be another relatively easy one to cover the Gulf of California WHS (from Loreto). If I do not make it to Baja California, I will "do" Islas Marietas from Puerto Vallarta for this tick.

I remember when we were looking at visiting the road from Mexicali to Puerto Peñasco (route 3) is actually in the core zone. For me that would have been enough to qualify a visit.
Though I will readily admit this would just be me in ticking mode, rather than getting the best out of a site, when it comes to natural sites I am a bit of a philistine and only really enjoy the ones with a spectacular wow moment, migratory/ birding sites are wasted on me I'm afraid.

There is an interactive map of the area here if it is of use in planning.

Or this one has all the Mexican protected area boundaries, showing that the Gulf of California and el Pinacate are contiguous, so if you drive from Puerto Penacaso to Mexicali you will always be in one of them

Thanks meltwaterfalls. Feeling more confident about that one.

I also have an update on one of my own questions:

It is possible to do all 3 by public bus, and just fly in and out from/to Mex City once.
For example: Hermosillo - Puerto Penasco (for El Pinacate WHS and optional Gulf of California WHS) - Ensenada - San Ignacio (for El Vizcaino WHS and Sierra Rock Paintings WHS) - Loreto (2nd option Gulf of California WHS) - La Paz. These are looong bus rides of course, but there are regular daily services and in reasonable comfort. For 3 out of the 4 WHS you'd have to join a tour anyway, so no point having a car parked somewhere. For the 4th one (El Pinacate) you'd have to succumb to a tour as well in that case.

I finished typing up my review for the Gulf of California WHS. It may have some useful information for you Els, as there are few other reviews. Water temperature at the end of November was 74 degrees (23 C), which was perfect (no need for wetsuit). This temperature will decrease some in December and January. La Paz, paired with Loretto might be a worthwhile area of focus for this WHS. I hope to return to visit other components.

Thanks winterkjm. I saw it this morning. There are a lot of WH people travelling in Mexico at the moment or the near future, there's a whole batch of reviews already scheduled to be published in December. I planned yours for Christmas Day - a special spot for a WHS that barely has been reviewed so far.

With a visit to Tlacotalpan today, I have finished my Mexico part of my 2022 Meso-America trip. After previous visits in 1997 and 2014, I added 13 more Mexican WHS to my score. This means that I ended up at 33 visited out of 35 Mexican WHS. The two missing are the faraway diving destination Archipiélago de Revillagigedo and the unfortunately-still-closed-due-to-hurricane-damage El Tajin.

For those contemplating a (first) trip to Mexico, find below my totally subjective rating of the WHS on a scale of 1-10. I will also write a post about public transport + do a blog post specifically on Northern Mexico as that was where the focus of this trip was.

1. Mexico City, Calakmul, Monarch Butterfly Reserves: 8.5
4. Oaxaca & Monte Alban, Palenque, Puebla, Chichen-Itza, Gulf of California: 8
9. Teotihuacan, Guanajuato, El Vizcaino, Uxmal, El Pinacate: 7.5
14. Paquimé, Xochicalco, UNAM, San Miguel de Allende: 7
18. Rock Art Sierra de San Francisco, Hospicio Cabañas, Franciscan missions Sierra Gorda, Tehuacán-Cuicatlán: 6.5
22. Sian Ka'an, Morelia, Popocatepetl monasteries, Tequila: 6
26. Zacatecas, Tlacotalpan, Luis Barragan House and Studio: 5.5
29. Querétaro, Campeche, Camino Real, Caves of Yagul & Mitla, Aqueduct of Padre Tembleque: 5

My 2022 trip to Mexico I did mostly by public transport, except for Baja California (where it would have been possible also, as I found out later). Here's an overview of bus connections to the WHS outside of the larger cities. Thanks also to Jarek and Shandos for their valuable input. None of the buses need pre-booking unless stated otherwise.

· Nuevo Casas Grandes (for Paquimé): 4 buses a day from Chihuahua with the company Chihuahenses. For this one, it is advised to pre-book at least one day before, as there aren't many buses and this stretch is part of a longer route people take (CDMX? to Nogales).
· Teotihuacan and Otumba (Padre Tembleque location): very frequent from Gate 8 at Mexico City Norte bus station (Otumba is the same bus as to Teotihuacan, just stay on the bus, it's the next town).
· Jalpan de Sierra (Missions): 4 daily buses with Coordinades from Mexico City Norte (7h). Onwards to San Luis Potosi (from where Zacatecas is an easy hop), 4 times a day at 5/7/13/16h by Grupo Vencedor (7 hours on a slow bus). Note: the bus terminal in Jalpan is way out of town, but Grupo Vencedor has a marked bus stop right in the town center as well. Tickets can be bought on the bus. There are also frequent buses to Xilitla (stopping in Landa as well).
· Tequila: hourly buses from the old bus station in Guadalajara (company Quick). Buses to Lake Chapala TWHS leave from there too every half hour or so (company Chapala).
· Yagul: from Oaxaca, I took an Uber to the site (400 pesos, Uber has only been active in Oaxaca since the end of last year, "normal" yellow taxis are deployed); for the return, I took a tuk-tuk (!) to the town of Tlacolula and a bus to Oaxaca from there (standing room only).
· Tehuacan: from Oaxaca there are almost hourly buses by Fypsa to the city of Tehuacan from the 2nd class bus station (Asbastos). They take 4hrs and drop you by the side of the highway. The walk into town is quite pleasant (has pavements, restaurants) and after a km or so enough minibusses will pass by to take you the final 4km into town for 8 pesos. The twice daily ADO bus has inconvenient hours. From the city terminal of Tehuacan, go to Miscelanea Terminal Zapotitlán to switch to a regional bus for Zapotitlan. This stops at Helia Bravo Hollis Garden. [if you are really pressed for time: the bus to Zapotitlan passes on the other side of the road when you walk from the highway towards Tehuacan, it stops for example across from the semi-roundabout some 300m from the highway]
· Tlacotalpan: hourly buses to Veracruz with AV and TRV. I arrived from Tehuacan via Veracruz – the most convenient is the direct ADO bus though it leaves at 3.34 am.
· Loreto (for the Gulf of California): 5 times a day from La Paz with the company Aguila.
· San Ignacio (for Whale Sanctuary El Vizcaino and Rock Paintings of the Sierra de San Francisco, although you need separate transport to the gateways of these 2 WHS, this can be arranged locally): 6 buses a day go southbound (Loreto or La Paz), and also 6 northbound (Mexicali, Tijuana and Guerrero Negro). Schedules are here.

Just to add something about buses to Jalpan de Sierra... We caught the bus from Queretaro, from the main bus station. There are buses with Flecha Amarilla about hourly each direction. The buses take from 3 1/2 to 5 hours (perhaps slightly different routes, we choose the quicker ones). It's an economy bus with no seat bookings, although queue early if you are catching the 9am Saturday morning bus at the start of a long weekend - there were people standing in the aisle most of the way! Our return bus on the Sunday was far quieter.

The bus will firstly drop at the start of Jalpan near the Mission, but presumably also continue on to the bus station (lost in our poor Spanish...) We recommend Hotel Santa Lucia, less than a 5 minute walk from the bus station. The buses on to Landa are at least hourly - both times we got lucky and waited no more than 10 minutes.

For the Cenotes TWHS, the list of the 99 cenotes that are in the Ramsar listing are in the appendix of this document: https://rsis.ramsar.org/RISapp/files/RISrep/MX2043RIS.pdf. This is the overall Ramsar listing, including a link to a map: https://rsis.ramsar.org/es/ris/2043