I think a good strategy is to change companies often and ensure that your side project isn't related to any of them. This creates a lot of complications and add

I think a good strategy is to change companies often and ensure that your side project isn't related to any of them. This creates a lot of complications and adds a layer of protection because if your current employer tried to claim the software as theirs, your previous 3 employers would all have claims (weak claims, that is). If the software is different from what all 4 companies would produce and it is all built outside of business hours, it gives you full leverage. If any specific company tries to imply that their claim is valid, they cannot do this without validating the claims of 3 other companies... Thus preventing themselves from obtaining the full ownership rights over the product. The company which actually wants your software would be better off just paying you and accepting your simple version of reality than trying to create complications for themselves by inventing some elaborate legal fiction.