This makes the template a lot shorter.
Overlays are not only harder to read and understand (especially if
multiple of them have to be combined), they also have a
real performance impact as each flake would introduce its own instance
of nixpkgks, which can further multiply if these flakes are than import
into multiple NixOS configurations:
https://zimbatm.com/notes/1000-instances-of-nixpkgs
If the goal of overlays was here to make importing easier, I would
rather recommend looking into https://flake.parts
