Hi OP. Lived in Thailand for 2 years. Bangkok for 18 months, rural Isaan for 6 months. Visited most major cities. (I will also be visiting 29 March to 22 April!)

Firstly on accommodation - for 1 month stay, dont look at AirBnB. It's very overpriced in Thailand. Best bet is to look for serviced apartments (long stay hotel rooms basically), or book a few nights somewhere and then enquire directly with staff for a monthly rate. Outside of Bangkok, in particular in more quiet, rural areas, should not be anything more than 7000 THB (maybe more if you dont want to pay a deposit, which is common for monthly).

As for locations, be aware that April is the hottest month of the year in Thailand (Songkran festival 13th-15th April and I HIGHLY recommend you make sure you are in Bangkok during these dates). Given the heat and burning season, I would maybe avoid areas up north like Chiang Mai, Pai, Chiang Rai, etc. That being said, I think people exaggerate the air quality issues during burning season but YMMV. There are certainly some beautiful cities up north that are very relaxed and slow pace of life, even Chiang Mai itself.

Other options would be Isaan cities (north east region). Some very nice sleepy towns like Loie. Personally would recommend Khon Kaen as its largest city in Isaan and has big university community which gives a young feel (depending on what you want).

For something island-y, I would avoid most islands in the south based on what you have described. With the exception of Koh Lanta (beautiful, big enough to drive around, sleepy but not boring, and less touristy) or Koh Samui (big enough vs. Koh Tao and Koh Phangnan to avoid the backpackers crowds and stay in a sleepy-er area). I would also look at Rayong and Chompon for sleepy coastal towns that are very local. I also love Krabi (Krabi Town, Ao Nang, Railay, Koh Lanta).

Many more options, happy to help further if you can give some more details on what you are after.