You might look into "joint ownership". You and the client(s) both can do whatever you want with it. In the contract, you might want to agree that the clients do not go into the business of selling the software to others, since that would compete with you. You need a contract, and a lawyer.

If you've finished the software and are not heavily customizing it for a client, then I believe that is more of a licensing plan. Just sell it. Again, lawyer.

I would not mess with a partnership. I don't have a good reason for this, but it seems to involve tighter coupling than I'd want.

Note: I'm not a lawyer (but I have sold semi-custom software to companies, without a lawyer; I would do it differently now but it worked out fine).