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!
And another one: I briefly checked your
mix.exs, which loads the:runtime_tools, which again require the:wxapplication.Remove that and your final app will not include wx anymore.