The buffet at the Westin on Sukhumvit was a favourite pre-Covid. I’m not sure whether they’re still doing it but it’s worth a try. I’ve also enjoyed the buffet at the Pullman Hotel down the road from soi cowboy. The Sheraton Hotel also was renowned for its buffet, although I have never tried it. I would also have a look at the Marriott hotel just passed Soi 4, they’re quite well-known for their buffet for breakfast and dinner, it is pricey though.