Lamai is not really all that different to Chaweng, more like a smaller version. Maenam is only a little up the road from Bophut so the only real difference there is the beach, Maenam in my opinion is a nicer beach.
Samui is not a big island and personally I would not move a third time but, if you want somewhere different then I'd be looking to the west coast, Lipa Noi or around there, not as developed and a contrast to the east coast towns.
Either that or just settle on the 2 locations you've already chosen in Chaweng & Bophut. Lamai & Maenam are both easily reached by Songthew in less than 15 mins (Chaweng - Lamai or Bophut - Maenam) for day trips to look around and enjoy their beaches.
I wouldn't be moving around so much, as Matt says it's a small island and hiring a car would be far more beneficial instead of uprooting yourselves every few days and checking in and out of hotels.
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