There's no visa for the self-employed so you'd still be stuck on a tourist visa if you don't meet the requirements of any of the non-immigrant visas.
To work you would be required to be employed by a Thailand based employer, yes. I don't know what you do for work but there are jobs here which foreigners aren't allowed to do:
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I believe that list is up to date but I'm sure someone will correct me if I'm wrong.