Possibly a short intro letter stating you are applying for the soft power Muai Thai training category. Don't know why you're including business related documents on a soft power application. You'll likely need proof of being in Cambodia such as entry stamp and hotel booking/address.
which visa doesn't matter, it's a numbers game. You can hoof it till you drop or get lucky, or get an agent in Bangkok, or take a morning trip to Pattaya for an agent to fix you up the same day
there is a law recently amended, and the proposal for another law more in line with globalist taxation economics. Don't worry about the law that isn't passed yet. The amended current one has to do with income derived outside of Thailand and brought into the country in the same or following years. My understanding is if you can prove the income was earned previous to January 1st 2024 you won't be taxed on it. Apparently the way it's written has a million loopholes to take advantage of.