Since GHA is free for public repos, and I took inspiration from nixpkgs-review-gha ( GitHub - Defelo/nixpkgs-review-gha: Run nixpkgs-review in GitHub Actions · GitHub ) which a lot of maintainers use including myself, why not just run all the build jobs on GHA? That’s why I made
It has a python tool to generate build matrix and bunch of inline shell script to set commit check status just like garnix would, and push to a cache endpoint (currently tested on attic, cachix, and I’m currently debugging niks3). It also completely skips paths that’s already in cache and sets status for them (excluded jobs also have status)
Users basically only need to copy pasta a yaml file and run a few gh variables/secrets set to enable cache and you would basically have garnix for free without 2 downsides I had with it (limits to 100 jobs, does not support legacyPackages.* matching)
Free build for everyone! No need to get a big server to build aarch jobs anymore 
EDIT: you can check the check status sample in the repo itself or GitHub - stepbrobd/inc: the second largest nix monorepo · GitHub. Click on the green/yellow/red dot/cross associated with a commit)