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.