treefmt-nix VS nix-beam-flakes

Yeah, I think you're right. I'm giving it some time to see what other projects people mention they think it's similar to (or ideally if it's a worse version of something that already exists). On the Nix discourse, someone mentioned https://github.com/numtide/treefmt-nix, which I think it compares similarly to as pre-commit-hooks.

I left it vague because I didn't want to seem self-promotional. Unmerged documentation branch is the best starting place here: README, docs folder of that branch, example from develop. I daily drive it at work, and on a personal project for about 8 months. Doesn't do well for monorepos with disparate BEAM versions yet, but there are escape hatches and usage-as-library too. For the typical Phoenix project with a few extra system deps it's a pretty good onramp. perSystem.beamWorkspace.devShell.extraPackages is how you add arbitrary buildInputs like your VIPS stuff and .extraArgs for additional arguments to mkShell, such as your non-sensitive envvars. There's a page for future goals too. Publishing the rendered docs as a static site is in my TODOs, it's mostly glue left.