Most embassys have been asking for Ltd company certification for gym for 3 months now. It's become a standard requirement actually. Congratulations on visa !!
we ( Muay Thai Fever) are running a promotion this month. 15,000 baht for 6 months. Ok it's a little bit more than what you paid but a 6 month course eliminates the chance of them issuing a tourist fever ( didn't happen to you luckily)... and well it's 6 months training too.
some people had success with short courses but more recently embassys are issuing tourist visa instead of DTV for short courses.
Ourselves,we only do 6 month course as this matches the length of stay, 180 days.
We don't want to have any failures so just stick to the 6 month plan which has worked for over 80 applications so far. At 15k I don't think it's too expensive