I do this for my rust box. Nothing but spinning discs for NAS. Boots from a usb stick with an immutable root. Persistence overlay partition keeps config and changes and updates and stuff. I think it was something like a partition for overlayfs defined to mount in grub. Or maybe the only grub stage customization was the wpasupplicant... Been a while since I made it. Something to load boot into ramdisk as well so logging and such doesn't thrash the trash silicon to death. Many ways to skin it!