Everything you said "should" is exactly what Nix and NixOS is.

Hey, I sorta did this. Lazy substitution, at least, not lazy build. Plus a bunch of tricks to speed up downloads.

https://github.com/dnr/styx/

Lazy build sounds tricky: you don't know the contents of the package before you build it, so you don't even know what to symlink into /run/current-system/sw. I guess you'd have to have some kind of wrapper. Maybe similar to comma.

I just solved that part by setting up CI for most of my system config (integrated with the above).

You're looking at the docs for "visibility". Visibility is a separate eventually-consistent data store off to the side that's used for certain queries so it can be scaled independently of the main data store, and indexed in fancier ways. The main data store for all the stateful and transactional stuff has always, and probably will always, support Cassandra. For visibility, the recommendation for high scalability is currently Elasticsearch.

Temporal may have properties that make it not a good fit for a particular use case, but scalability is really not one.

Temporal can also run on Cassandra, which scales much larger than Postgres (if you put in enough effort). It can also be replicated across regions for high availability. It's already running some pretty huge use cases.

(I work at Temporal)

I'm also using NixOS and working on Go projects, and had to deal with out-of-date Go releases. Nixpkgs generally does get the latest Go versions pretty quickly, but only in the unstable channels, they're not backported to NixOS releases. You can just grab that one package out of nixpkgs-unstable or nixos-unstable, like:

    (import (fetchTarball "https://github.com/NixOS/nixpkgs/archive/nixpkgs-unstable.tar.gz") {}).go_1_21