It's been years and years since I did any of that and I don't remember all of the gyms. But back in the day, I would just hear about a gym, get a taxi or tuk tuk to take me, ask a trainer when I got there if I could pay a drop-in fee, then go at it. The only draw back to this is that from the gym's perspective, I was a,"here today, gone tmorrow" type. But I'd ask questionsx or ask if someone could hold mitts for me. If I thought I got a fair shake, I'd keep coming back. There was one gym I stayed with for a full month in Bangkok once. They treated me very well and let me spar with their up and comers. Damn... This is almost 40 years ago. That gym is long gone now. But one of the trainers there made his own gym which caters to foreigners. Some like that, others resent going to Thailand just to be in a Muay Thai gym surrounded by other foreigners. I kind of agree, but to each his own.
(Here's a discussion from 3 years ago about what it's like at his particular gym, by the way.
We over 50 guys have to take the few advantages life actually affords us.😅 One of them is that we have more visa options than the kiddos get. Muay Thai is fun. I first joined a gym and trained with the regular guys for both morning and late afternoon sessions. When they pair you up with a skinny 11 yo who can't be more than 80 lbs, don't underestimate the lesson you might be about to receive. I learned so much about head control and leverage and timing from this small fry who jerked me around willy nilly, but then would pause to show me how to stop him or how to do his moves and throws, myself.
I'm pretty sure gyms(the qualified ones) will take your money. Whether they train you seriously or not is another question.
You are also eligible for the retirement visa, by the way. Have a looksee at the link below. Then you're not tied to any one gym for your visa and can move on from one if it isn't working out for you.
Your adventure is terrific and he's just an imbecile who can't even understand the Shakespeare quote he misused. Pedal on! You're inspiring many of us!