Yea, i actually do that for a couple of my dependencies. My problem is there are dozens or hundreds in my OS that instantly upgrade if i change my `nixpkgs` ref. And i have no clue what version _any_ of those are at, off hand.
Eg lets say i want to fix my X - because when i updated my `nixpkgs` pointer Xorg was using a lot of CPU for some reason. There's no lockfile that says what version X is at, or what dependencies X is using or associated with. Is it one package? Is it a half dozen? I could open the repo and dig through the configs to locate it and _try_ to infer what the hell version it's at, and what dependencies it has.. and then find what they're at and so forth - but it's convoluted imo. Much easier in Cargo.lock, for example - something i've done dozens of times before.
In short, basically everything in my OS broke when i updated my nixpkgs ref, and it's non-obvious and maybe non-trivial to understand what versions everything is at. To incrementally try to un-screw my system when i change my nixpkgs.
As i move forward, any packages i install i'm tempted to bind them to very specific inputs. So that when i want to upgrade one thing, only that one thing changes. But this workflow seems difficult in NixOS. Despite it being the workflow that i want.
Well, maybe not difficult - but lots of LOC. Adding dozens of inputs just to change versions of stuff without breaking other things seems like a lot of work for a dependency management system.. imo.
I have a script which I use [1] after updating my Flakes to do just that : know what has been upgraded.
[1] https://github.com/nix-community/nur-combined/tree/master/re...
Also, I usually manage my home packages separately from my global config. The easiest way to do that is probably by adding a different channel/path to your user’s nix-channels. I believe home-manager uses that. Alternatively you can also do the trick I mentioned in your home.nix file.