If you are doing this every year then your chances of being denied entry increase. It's not a good plan for living inside Thailand 6 months of every year
Anonymous participant until you have a written plan signed off then you won't get the DTV for medical reasons. It's all going to depend on the doctor and hospital being willing to play ball
just to clarify you pay the Muay Thai course fees to the gym you will use in advance and the gym then provides the paperwork you need to apply for the DTV
Taking injections every month won't get the visa, you could do that wherever. You'd need a treatment plan for a number of months (6 or more) signed off by a recognised doctor at a hospital