Is proper support for arm64 included in nixpkgs, and also is there some native / built in way to declaratively create disk layout?
Not sure what you are expecting by “native / built in”, but there is GitHub - nix-community/disko: Format disks with nix-config [maintainer=@Lassulus] .
Included in nixpkgs is what I mean
Yes, but there’s no guarantee on the software support.
Not really.