You have quite a heavy schedule there. Some of the places I have not been to so I can't comment.
If you are going down as far as trang then a visit to lipe may well be worth it. Koh Lanta is a lovely place then the natural flow is to Phuket and Phi phi.
If you are going to samet you could do your last night in pattaya as it would break your journey and with the new road it is only about an hour to the airport. It can take you that getting from the centre of bangkok.
Accommodation wise I am not much good a s we tend to go a little more up say 3 to 4,000 baht a night but I would recommend that you look on the sawadee web site as there is all kinds of accommodation and loads of reviews.
Happy hunting

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Kiss
God laughs at people who make plans.