its difficult to get the balance of a non-commercial resort. That will mean you may not have all the facilities you want. I stayed in Thailand for 9 months and then returned last year for a further two.
There are some out of the way resorts past Hellfire Pass?
What type of holiday do you want? beach, trekking, a mixture. Koh Chang near the Cambodian border is nice and a popular place with thais.
Let me know if you want any specifics
PM if you want