I have promised to myself not to deal with these hotspot issues any more because the rules are anything but clear and consistent, but I can share my local (Finnish) view.
Saint Petersburg hotspot –> Suomenlinna:
This is tight for four hours time limit, because from railway station is a tram ride or walk to the ferry port and then 15 minutes by ferry. Maybe it is barely doable.
Helsinki hotspot –> Saint Petersburg:
I think this should be in Helsinki hotspot, because it is very easy trip to the core zone. Back in the days before covid, lots of people did day trips from Helsinki to Saint Petersburg (for example for watching ice hockey games). And with a new evisa this should be even easier and cheaper.
BUT was there a 200 km limit, or 250 km (not 300 km)? Then this Saint Petersburg / Helsinki is out.
Always a bit reluctant to count taking a cab. Doesn't feel like a simple hotspot.
Since private/rental car also counts, why not taxis.
If we cut taxis I don't think we can have a Helsinki hotspot as their struve site requires a taxi (unless we can add saint Petersburg to it)