I’ve continued to work on this project (with some additional info on my biggest hangup at BTRFS Pi won't boot from compressed subvolume - #5 by n8henrie).
I’ve rewritten the project and encapsulated almost everything into a flake so if any reader are interested they should be able to build the project themselves: GitHub - n8henrie/nixos-btrfs-pi: Raspberry Pi SD card image for NixOS on BTRFS root
I’ve also configured a GitHub Action to automatically build and upload the resulting image as an artifact (which will expire after a few weeks IIRC): https://github.com/n8henrie/nixos-btrfs-pi/actions/runs/2924556027