So, according to a number of reliable and different sources, although getting an "extension based on working" on an ED visa is technically possible (1.), the issue is that the work permit approval letter is usually refused.

 

I wanted to work as a teacher at a private/government school. However, if I can get the WP for a language school, or even a corporate job, that would be an option. I was told that for these kinds of jobs, it would be much more likely.

 

Is there a better chance, for an "extension based on working" with an ED visa, to get the work permit approval letter accepted, for a language school, or a corporate job?

1. https://www.immigration.go.th/en/?page_id=1890 (6. and 7.)

Ask your local department of employment/labor, this might vary from province to province