Sure there is no silver bullet. However, I don't know about anything which would since this flathub problem as well as Nix:

* Works on all (most) distros * Builds the packages from sources, binaries are only a cache to speed things up. * Bonus: already has a lot of software packaged

And proprietary works in the system too, the build step can just download the binary from internet. It's not a preferred way to do it, but you don't have to build a broken system just because of few packages without source access