What are the best visa options for a 6-month stay in Thailand?

For a METV you will need to provide a bank statement showing a deposited amount of money on it (i don't remember exactly how much is it, it should be around 6000 - 8000 euro). If you can't provide this statement, the easiest option will be to obtain two SETV with an extension each (2+1 × 2, 6 months of permitted stay). You will have absolutely no problem getting two back to back tourist visas. From Chiang Mai you can go to Ventiane, Laos, where i had the smoothest overall experience. Don't mention muay thai training, just state, if asked, that you are on a 6 months holiday.

two options: Single Entry tourist visa give you 60 day's, extend once with 30 days than leave the country, apply for new tourist visa for 60 days, extend 30 days than leave the country and come back with Visa Exempt Entry 30 days or apply for the Multiple Entry Tourist Visa, get 60 days of stay at arrival, you can extend wit 30 days (than leave the country) or leave the country get 60 days again, Just before valid until date leave the country and get 60 days again. That covers your whole period.

Get a Multiple Entry Tourist Visa (METV) for unlimited entries during 6 months. You can even get another 60 days at the end of the validity period.

OR if METV requirements are too harsh, get a Single (SETV) for 60 days then extend 30, then get another SETV at neighboring Thai consulate and repeat. Add a quick border bounce at the end for the last 30 days visa exempt.