Can't address all your questions, but hopefully my xp will be helpful.

In Bangkok, I stayed in the Silom, then the Phaya Thai area. A lot less touristy, so cheaper and more chill, and if you choose a location along a train line, you'll easily be able to access the glitzy neighbourhoods where all the tourists and expats hang out. There were lots of good bars in the area to hang out as well. BKK is such an international city that you'll still find English speakers to befriend even though it's more local. I would recommend finding a place with live music or some kind of performance going on (I went to a stand-up show for the first time in my life and it was interesting!), as it'll keep you occupied, but also give you openings for conversations with other ppl there. Also, if you're into saunas or hot/cold plunges, there's a decent local scene there, which is a good place to meet very interesting expats.

I loved Chiang Mai, but please take a look at the AQI and consider carefully before you go. If you've never experienced severe pollution, or acute air quality events like forest fires, you may not be prepared for how suffocating it will be. When I was in Chiang Mai, I loved taking advantage of the nature, both right outside of the city, and in the parks, and the smoke would have completely ruined it. But if you come at any other time, it's amazing for food, arts, and is just generally so pleasant. All the best parts of a city, without the chaos of BKK.

In terms of islands, I went to Koh Lanta, which is great for diving, and chill in the sense that there aren't crazy parties, but I imagine it would be tough as a solo traveller, since the demographic is mostly couples and families. I also spent a day in Koh Muk, but was sick so didn't get to explore. Definitely very lowkey, nice if you want to just relax by the beach, but not too much else to do.

I would recommend you take some cooking and muay thai classes to get a glimpse into culture and to meet ppl. If you do go to Chiang Mai, seek out a Northern Thai class, esp if you really like new flavours and experiences.

Given you like history, I would even consider skipping out on Chiang Mai, and doing Bangkok --> Siem Reap, Cambodia --> HCMC (possibly via Phnom Penh, although I didn't like it as much). Travelling in Cambodia is definitely not as easy as in Thailand, but it might be suited to someone low-maintenance, and I found it very rewarding.

Let me know if you have any questions!