This was based on content found at https://programsareproofs.com/articles/react_native_nix.html. It was a great starting point, but didn't work for me. I run PopOS with Nix managed by home-manager. Sometimes, things that work great out of the box for NixOS users don't always work for me, but this is what did. (I'm not sure if the original author uses NixOS)
I am using devenv as well.
I needed to bump the inotify values on my machine to make watchman run without bailing with an error immediately.