Working online in Thailand but with a company outside of Thailand and not effecting thais in any way has always been a Grey area. It's technically illegal but most definitely tolerated. The muay thai visa is currently easier to secure than the nomad one. Just use a gym which is a Ltd company and registered with the government
We have found that a few embassy were accepting non soft power activity such as scuba diving, self defense initially but slowly they have began to gravitate towards insisting its a soft power activity. I run a muay thai school and have processed 50 so far.
The way we do it (muaythaifever.com ) is we sell a 6 month course of 1 lesson a week for 6 months. We write it up so it's a weekly class. But once you have the visa its flexible
Can do 2 class a week for 3 months or just do them all in a month. I think the Thai Government wants people to travel nit stay I one place and in one gym