I’ve been here 7 months & my main reason was training MT
I would avoid on site gym accomidation, a lot of them are fairly dirty & noisy during the day so its good to give yourself a mental rest from the place, food is cheap so dont ever get meals with the gym or you will have to eat at a set time every day.
North thailand is half the price of south thailand, south thailans feels overly commercialised, very “touristy”. however the north is very quiet at night to & most places shut fairly early. you ned to try 3-4 gyms in each area & see what works for you. dont book anything long term up front