Every embassy has different rules. There is no set list of docs. The requirement ìs that it is a professional course organized by a government registered facility and employs certified instructors.
Re muay thai, there are various government bodies that certify instructors and gyms so best find a gym that ticks every box just to be on the safe side.
Sports authority
Board of boxing sport
Education ministry
All above issue government recognized certification but the embsssys might demand one that the gym doesn't have. By no means does that mean the gym isn't legit ... it's just one of those things
Most embassys are now asking for a Ltd Company certificate too.
We learned this early on and acquired all the certification to tick every box. We lucky in that our coach is on the governing panel at the sports authority and helped us get covered
yes its been brilliant my gym has a lot more customers from the DTV applicants and it's great to be introducing the sport so many new people. The soft power initiative is great
Embassys are changing and looking for longer courses. When we did our first one (I own a muay thai gym) 10 private lessons got approved. But now increasingly they are looking for a 6 month course to match the 180 day stamp. If you submit a short course it could get by but there is a chance they could say you don't need a dtv a tourist visa will suffice. So to be on the safe side choose a longer course.