This is actually a bit of a match made in heaven for Nix. The Steam Deck runs Arch with immutable root, which means most of the AUR packages won't entirely work. So you need a package manager which respects immutable root, supports atomic upgrades with Arch and has a package selection similar to the AUR. There are a few options, but the best one really is Nix: https://repology.org/repositories/statistics/newest

Given that package management on the Deck is traditionally handled with Flatpak, Nix seems like a great alternative for power users with storage to spare.