Currently I have this similar use case, I have a PCI-E NVMe that is not
detected by BIOS (KGPE-D16 motherboard) so my plan was had a partition
for /boot in my SSD and the root system in the NVMe, so could be great
have support to that user case that I think is pretty common in old
board that doen's have NVMe and you want extend your system.