Still continuing to tinker, but at a slower rate.
I think the current iteration is pretty satisfactory. It has options to put the boot files on FIRMWARE, which allows booting from a data=DUP BTRFS root (maybe handy for SD cards? I image should also work if you had a BTRFS RAID setup), or to boot directly from a @boot subvolume if you don’t plan keep data=single.
Never figured out this issue with zstd compression on @boot screwing up u-boot.
Current iteration: GitHub - n8henrie/nixos-btrfs-pi at 0b33a340eea39aa8642ef9f6223aeebbf3d16316