I think this is a good point -- The ideal state is probably something that is immutable where you need it, and mutable where you may need to make frequent changes. Making small tweaks to your configuration should be easier than needing to write and re-evaluate Nix expressions.

We took a similar approach with our product (Devbox): Immutable package store (powered by Nix) to guarantee correctness, coupled with mutable project state. Here's a post where we explain our thinking: https://www.jetify.com/blog/do-repeat-yourself-global-packag...