Have a valid and genuine passport, have the visa printed out, have the TDAC done.
and if asked just talk with the IO about things that matter.
as seen earlier, your private life, partners, previous visits, the purpose of being in other countries are non relevant but may be nice to talk about with a coffee at another place.
IF it is really 210 days and not the normal maximum 180 days, you have to get that stamp corrected.
Thanks for ignoring those who tell ignore without any advice or knowledge about visa rules. some are angry because they ask advice but not like to hear the rules.
The DTV is valid for 5 years and that is way to long for your plans.
Apply for the Multiple Entry Tourist Visa which with good use cover the time period of your Muay Thai training course.
Get back home, finish your job, set up a new on-line company and when it brings revenue, apply for the DTV workcation and you can move to Thailand to train in your spare time.