Given that you already have flakeified everything, it's not that hard to pull in a different version of nixpkgs and cherry-pick packages from it. I have a (somewhat complex) setup utilizing this: https://github.com/casept/nixos-config
Yea, i actually do that now. My problem is the dozens of packages i didn't hand pick all broke. So basically i need to hand pick everything. And in doing so, it feels like a lot more work than simply specifying what versions i want during the package install.