Maybe go for a license / ??? that allows for free use but requests payment for paid use

Actually, that would make it non-free (a.k.a source-available) software by FSF's definition. You can do something like Qt (copyleft license for free, permissive license for a fee), but this doesn't really work for a lot of software.

I tried to get paid consulting from it but never got a penny. But you're right, if I were more aggressive it could have worked as supplemental income.

Eh, that's a lot of work. It's no wonder why there's such a large "Malware as a Service" industry nowadays