You may find something like node2nix helpful (https://github.com/svanderburg/node2nix). This converts your package.json into a Nix expression that can then be cached. You're right that it does require some setup and a bit of Nix knowledge but could yield significant benefits and take a good chunk out of that 20 minutes.

Another option might be to use pnpm instead of Yarn and cache your pnpm dependencies. pnpm actually works a bit like Nix in that it creates a pnpm-lock.yaml file with content-based hashes for the full package.json dependency tree. This enables it to quickly determine which parts of the dependency tree it needs to build and which are already available.