@Jo *********
I can't see how someone would be caught for teaching English online from Thailand provided they do so from their own home. I suppose that could be considered work though, but is up to the interpretation of immigration. As soon as this activity takes place in public (which is a stupid thing to do) then yes it can definitely be considered work.
Also, a lot of foreign YouTubers do videos on Thailand. Some even warn about not working in Thailand, while they are technically doing so!
I'd say, making a few phone calls and sending emails, doing research for a foreign company, having nothing to do with Thailand should be OK. Just do it from home. Never from a Cafe or public place (although picking up a work call is OK, you can't be that paranoid).
Everything else, including YouTube videos - even if they're positive and all, technically it's working.