BTW - all this is likely to be different in the current COVID situation. ATM all visas are suspended I really can't see the Thai government, which is being extremely cautious, offering new Muay Thai visas with the physical contact that involves.
I applied in the UK and they are very strict about the criteria now though - I had my bank statement returned because it wasn't showing the last month's income.
METVs are only issued in your home country and can last up to 9 months if you do extensions to each 2 month stay. So you'd have to fly home to get a new one anyway. I've never stayed in Thailand more than 9 months in a year but have never had a problem getting a new METV.
The UK Thai Embassy is currently shut to foreigners. But to get the 6 METV you used to need (I did this Sept 2019): return plane ticket, £5000 in a bank for at least six months prior to application, written evidence of your income, ie letter from your employer or similar (I'm freelance and an investor so I wrote my own with documented evidence which they accepted), accommodation booking in Thailand for your arrival and £150 fee. You must apply within 3 months of travel. However, this was all pre-COVID. Post-COVID, who knows what visas are going to be permitted or what the requirements will be but my guess is you will need (at the very least) health/travel insurance and a health certificate within 4 days of travel confirming you are COVID-free.