immigration is against Chinese at the moment because of them doing dodgy stuff with visas, so very hard to get Muay Thai visa for them. In Pattaya anyway, my girlfriend is Muay Thai visa agent and cannot get one for Chinese nationals
There's a Muay Thai visa which means you can stay for 1 year (3 monthly check ins) but main reason is to study/train Muay Thai. Some available in Phuket I think, I can ask my girlfriend who works for the Pattaya Muay Thai visa team if they have a connect up there.