I made a direnv-like utility which hooks into your shell to allow using Nix flakes for dev envs / dev shells anywhere:

https://github.com/dfrankland/envoluntary

This helped me bridge the gap between installing packages declaratively via NixOS / home-manager and defining them for each project being worked in via flake.nix / direnv / nix-direnv; which was needed since most projects don't use Nix.