Rayavadee beaches are beautiful but yes, public access. That said, during my last two visits the interlopers were well behaved and seemed to consist largely of beautiful French models
so it wasn't as irritating as it might otherwise have been lol. With covid restrictions ending the crowd might degenerate a bit. For activity and excursion options it can't be beaten.

I found the remoteness and privacy of www.pimalai.com to be supremely relaxing. A huge private beach and it's very close to Phi Phi and other island excursions.

The price of isolation is a transfer from Krabi airport to Koh Lanta requiring 2 hours of land and speedboat travel, however, a cliffside villa with private pool overlooking the Andaman at the end more than makes up for it.

The ability to have a chef prepare a meal poolside at your private villa is a very nice experience I haven't often seen offered.

In the evening the spa and its surroundings are surreally beautiful. I've yet to surpass that spa in Thailand.