Other idea:
Stay with SaaS, real devs, real core product, closed source, but each customer can (if they want and pay up) literally skip multi-tenant and being on the same codebase as everyone else, and get an interface to actually customize their own version to their liking.
Remove unneeded features, change UX, UI, add features. Some dev spends tiny amounts of time ensuring nothing gets too crazy, but apart from that it's basically an autonomous fork of the product, continuously tracking main.
That will probably come a lot sooner.