I've only been doing this for a few weeks, so too early to tell if it's a good setup, but I added a GitHub Action that rebases my personal fork atop nixpkgs-weekly. I'm hoping that will help keep me from having a stale-by-default personal nixpkgs. (I use a personal nixpkgs to stage PRs waiting to be merged upstream.)
https://github.com/appsforartists/nixpkgs/commit/769a72d3a6f...
I started doing it this way (auto-rebase) when I started sharing one home-manager config across different devices.
This lets me do `nix flake update` on my home-manager config to have all my in-flight patches + the canonical nixpkgs from any device, and trust that they can all see the shared GitHub to sync. Hopefully will make updating less of a chore.
Only time it's bit me so far was when godot3 was broken in nixpkgs-weekly but worked in 25.11. Forced me to go write a PR for that and get it upstream to get my build working again, but that was more of a nixpkgs-weekly problem than a personal fork one.
One of the wrinkles of getting home-manager going on a bunch of different devices is that it liked to copy my local git checkout of nixpkgs to /nix/store a lot. That's why I'm preferring to have flake.lock point at my github.com branch, and then I can test uncommitted changes as-needed by passing --local to my home-manager switch incantation:
https://github.com/appsforartists/device-config/blob/c09d6bc...
I remember thinking about doing that, but found value in having my fork tell me when was the last time I synced with upstream which was something useful to assess if I wanted to rebase my patch once again. Eventually my change got upstream and I stopped tracking my own fork though.