Does only /boot have to be on a bootable partition?
Yes, I think you are intending to do something very similar to what I did a few days ago, and it works for me :-) See also this link.
But you need a few more things too, that should be on a bootable drive.
I installed an nvme drive, an m2 stick via a PCI Express card into my workstation, a Lenovo Thinkstation C30, that I bought second hand (refurbished). This solution [with an m2 stick via a PCI Express card] is much cheaper than buying a computer that is new enough to boot from nvme.
The nvme drive works and writes 2.4 times faster than my SSD connected via SATA. It can probably be faster when cooperating with newer interfacing hardware.
In this configuration it is not possible to boot directly into the nvme drive. The BIOS/UEFI system does not recognize it. But using Ubiquity in Xubuntu Focal I can make a boot partition in a SATA drive and from there boot into the root partition in the nvme drive. I can also run
sudo update-grub from my main operating system in the SATA-SSD to get a working menuentry.
In the Ubiquity installer of Xubuntu, in the partitioning window, select 'Something else' which means manual partitioning.
You normally select only the root partition, /, but you can also [create and] select a boot partition, /boot, and for example a /home partition. In this case put the root partition on the nvme drive and
/boot on a bootable drive (and you need no separate home partition).
You should also put the bootloader on a bootable drive,
- in the old BIOS mode alias CSM alias legacy mode, put the bootloader onto the head of the bootable drive
/dev/sdb or ...
- if a GUID partition table (GPT) you need also a small
bios-grub partition in a bootable drive
- in the new UEFI mode you need an EFI system partition in a bootable drive. If there is already one, it will be used also for the new Xubuntu system that you install. Otherwise you must create it and there should be a FAT32 file system. Xubuntu's installer will put the EFI system partition onto the first available drive automatically (usually
The following link describes the partitions that are needed in different cases,