You could still move e.g. /nix to the BTRFS disks and only keep /boot on the thumb drive, no? That’s more or less how I have NixOS set up with ZFS. But your solution is of course much fancier :D
Could you share more about your NixOS setup, or good resources? Genuinely curious as I’m looking into building a small, low-budget Plex server running NixOS :)
I started using hd-idle to spin down the drives when idle for 30 minutes even.
I do however run my system with root on tmpfs and have various data persisted on the SSD.