This is based on nixpkgs commit 862277ac9d34273cd953f42061e23d488d6b7e8b.
The resulting image is less than 25 MiB compressed.
The actual crosslink closure got shrunk from ~180 MiB to ~60 MiB just by using beam_minimal rather than beam as the base set in beams.nix!
As I have trouble following discussions on a gist, I'd suggest to move any further questions to the official discourse, which I can more actively watch, and where your problem is also more visible to other users.
https://discourse.nixos.org/