90 day is just a simple address report that everyone must do, it has nothing to do with DTV or documents … it is a 2 minute procedure (except the waiting line).
To my experience and understanding you can come back no problem and only after 1 year you likely need to show documents again (have’nt done 1 year yet)
apart of money, i assume they can ask why do you want to stay and extend if the initial muai thai course ended already … do i have to book a 6 month long course every extension ?