I've been very happy with nix. I've been using nix in the reciperium.com monorepo, granted, it's only me, but I'm quite happy with having everything there. From docs, to the infra with terraform, to frontend and backend. The procedure for the CI is quite straightforward (nix build .#project), and caching the dependencies in the CI works quite okay. Even the secrets are there, encrypted using age (might not be the best, but good enough).