Do yourself a BIG FAVOR...get the DTV visa in the USA before you travel to Thailand in September.
Otherwise, expect to be questioned by Thai Immigration Officers upon arrival into Thailand and possibly DENIED entry. Indeed you will be on their radar for trying to enter Thailand again as Visa Exempt Entry.
It's clear that Thai Immigration Officers are cracking down on those who like you, use Visa Exempt Entry into Thailand multiple times.
then it was possible to get a Thai bank account opened with the DTV visa. Now, it's NOT possible. And those who have been able to get the bank account by use of the DTV visa, may find their bank account closed. It's happening with Bangkok Bank account holders
be careful with the DTV visa for Muay Thai training. It's obvious that the Thai Embassy's or Consulates are stricter now than before. Best to get a Muay Thai training school we with at least 6 months proof of enrollment with a gym that has been successful in providing the required DTV visa documents for the application.
Hopefully you are planning to actually participate in the training.