It sounds like you’ve done a lot of good initial research.
With your background I would say you would be best served starting in a class like TaG and then moving into shifters later. TaG is a super quick class, but it seems like you’ve got a good enough racing background to not to get into too much trouble with it.
Most chassis that are suitable for TaG are also suitable for shifter classes. It just takes a few add-ons like front brakes usually.
With your size, a 30mm chassis will probably suit you better, but it depends on how much ballast you’ll have to add for whatever class you’re running. Your best bet is to figure out what the local shop is supporting for chassis brands and start there. Almost every manufacturer builds a 30mm and 32mm version (there are also 30/32mm hybrids out there) of their karts, but some brands (Tony Kart and it’s variants) are known to be softer and more flexible, that may play into your decision as well. It’s important to keep in mind that your body weight is super crucial in how the kart handles. Moving around in your seat and using your body in different ways on-track can really affect the kart’s attitude. So even though everyone has to be at a minimum weight, someone who is smaller has less weight to throw around and has to deal with all kinds of ‘dead weight’ on the seat. I’ve been in the same boat my whole life, I’m 130lbs now, and I have around 60lbs of lead on my kart to hit the 355-365 weight for TaG. The 30mm kart lets you induce more flex into the chassis through your body and it will react a bit better for you.
I can’t advise much on the local NorCal scene since that’s thousands of miles away from me, but it sounds like you’re off to a good start with visiting local tracks and shops. That’s what everyone is going to tell you to do on here anyway, as it really is the best way to get an idea for what’s going on in your area. Karting is very regional in regards to what engine package or chassis brand is supported.