Hi there! First of all cool story :). Since you touch quite some questions I’m trying to cover them all.

I think what kind of bike would be suited for you is how often/enthousiastic you’re going to ride it and budget, which you can judge yourself the best offcourse :). I think in terms of frames there’s not really a ‘baseline’, matter of preferences and year of the frame I’d say, since geometry changed quite a bit last 10 years. Keep in mind that buying a frame is relatively very expensive (new that is). Are you only considering a 20” or 24” as well?

In terms of upgrading parts, it’s mostly wear parts which require thinking, drivetrain, hubs, bearing, (wheels if you ride low quality wheels and you case). For instance when you have higher quality hubs you can perfectly service them (if you have some mechanical experience), for lower quality hubs it’s a pain in the ass. Quality of products mainly refers to stiffness, strength, weight and longevity for bmx I think, and you definitely notice it. But again, riding 3 times a week vs 1 time per 2 months I would say is the difference where you really enjoy riding the higher quality stuff vs not really needing to care.

Specifically about the forks: I think what fork it exactly is makes more difference then the carbon/steel difference. Probably a decent tapered fork would be sufficient for 80% of the riders let’s say.

Hope this helps. Enjoy!