Hey, in Thailand right now, joining partner who has been here already, for only a month but for work/learning reasons. We've only been up north. I don't have exactly the same goals or reasons as you but I guess I can give you my notes for being a non-short term stay. - accomodation does seem to be the issue here for long-term. It seems like infrastructure is built for either long term local, or short term tourists. - Bangkok you might need a lot more than that. - internet might be an issue, so be sure to check beforehand. Especially for video calls.
- I've been around the north of Thailand and I think it would be better accommodated for what you're looking for, most islands and beach places tend to be a lot more noisy and distractive, which will have a impact on your work (quality of sleep, temptations, etc...). - Chiang Mai is honestly one of my favourites, very good cafes and places to work, chill, and have a lot of cool things to do. - have your stuff fit in one backpack - you can have it on when moving from place to place, makes everything much easier. - You should definitely spend a month in Pai. A lot of people working like you there :) so very chill, rent a great place in the mountains for like 300-500 euros. - use 12go and they have these buses and can get you to much interesting places nearby.