People may also wonder why I drive (almost) past Kruger NP, but somehow a 'general' wildlife park doesn't appeal too much to me. I'd rather spend my time in isimangaliso & drakensberg.
We have visited Kruger 3 times as well as Isimangaliso (Once. It was called St Lucia in those days!). Kruger is ENORMOUS and you really need 3 days (plus)! to take in where the game is showing best across both the Northern and Southern areas - it is not a place to nip in and out of on a day trip other than simply to have "seen" it!. We always camped with our own tent which gave flexibility and placed us well inside the park at both dawn and dusk to maximize time on the trail (getting out onto the road the instant the rest camp gates open is important) and be away from the crowds. You would need to use a rest camp rondeval etc rather than a hotel outside or you would waste a lot of time if you kept going in and out. So, given your general timescale and priorities it is probably best to give it the miss.
There were of course no WHS for any of those visits (RSA's first was as late as 1999) and a lot of the Veldt area really has very little to attract apart from those so I agree it is best to reduce time spent on them to the minimum required to tick them off!
I don't know how "interested" you are in birds (or how well equipped you are to see them!) but, in all honesty, they are Isimangaliso's main attraction - Kruger of course lacks the water-based habitat. It used to be said that nearly every South African (and, in those days they meant "white" South African) home would have a copy of Roberts "Birds of Southern Africa". It is a very special bird book and could be a nice souvenir. It is cheaper in RSA than outside unless you could get a 2nd hand one in NL. Reasonably good binoculars are well worth having for viewing mammals but are really essential for birds.
RSA has been trying to "beef up" Isimangaliso in recent years to provide a more "general" wildlife experience and have re-introduced Lion and Elephant, so, in theory, all the big game is viewable even if a bit "zoo like" (We particularly felt that nearby Hluhluwe sufered from that impression). See - http://isimangaliso.com/newsflash/lion-king-returns-44-years/
Our last trip to RSA was in 1998 and no doubt the wildlife experience in all of the parks will have changed beyond recognition with vastly increased numbers of visitors which can't be good news for game viewing - but, as everywhere, you can only take in what is available nowadays! If WHS wasn't a factor and birding wasn't also a major objective then Kruger would certainly have provided the more "authentic" big game viewing. (Though we found Kgalagadi wilder still)
On an earlier trip (1986!!) we also did the jeep ride up the Sani Pass and into Lesotho - and also stayed at Himeville - the Sani Pass Hotel in a cottage for R49 pppd FULL BOARD!! (We remember it as a very pleasant experience but, as an indication of "how things were" in those days, I have just looked at the brochure which states "Servants (Non European) R9.00 daily. Own bedding required" ). The excursion was quite spectacular and provides a new country "tick" although it didn't really provide much of a "Lesotho" experience apart from the local horses and blanket sellers! (we went into the centre of Lesotho from the other side on a different visit). The road was VERY rough in those days and really needed the jeep .
There is a lot of good walking in the Drakensbergs and I think it is wise to give time to both the northern and southern areas separately. On my very first visit to RSA in 1979 I remember doing a walk up to the Tugela Falls - a treasured, if fading, memory!
Other things to see in your chosen area?
a The Voortrekker Monument in Pretoria is well worth a visit to get a feel for "Afrikanerdom".
b. Pilgrims Rest and Barberton. Both tied in with RSA's mining history. Rather strangely, the Barberton area is only on RSA's T List for natural aspects - including its gold. If you had time the direct road over the mountains from Barberton across to Swaziland would get you another country and also show you the modern reality of mining. Then carry on south to Mbabane and on to the Drakensbergs?