I like your idea. Very realistic and do-able. I like your idea of a local hotel and just getting the best room. 2000 baht is a good budget for that.

Anyways, I haven't been to too many islands/beaches, so I can't tell you where to go. However I can tell you a few places that I didn't like (for a romantic trip)
- Krabi. I stayed at Ao Nang. Ao Nang is a place for Europeans (almost exclusively when I was there). The only Thais I saw were the workers.
- Phuket. I stayed at Kata beach with a special lady and it was great. We also stayed at Patong and I would not recommend it.
- Koh Chang. I stayed at Lonely Beach and ended up leaving the island a day early because Lonely Beach was so bad! The workers at the bars were caucasians! Koh Chang was really nice...just have to find the right beach.
- I liked Ko Samet, but you just need to find the right beach.
- I want to try Rayong sometime.
- From what I hear, there's a lot of expats on Samui.

I hope I at least gave you something to think about. Your idea is good, now you just need to find the location.