so your NVMe is listed as /dev/sda or something else?
have you tried to use no raid at all during installimage setup - instead of commenting out the 4TBs…
if it is sda you ideally could set swraid 0 (for no raid) and declare the partitions as you need it, which should only use the nvme drive then anyway.
ideally the system starts with that and you can create your sw-raid for the 4TBs afterwards easily.
if the nvme is not sda or the system does not come up properly, I’d comment out the 4TBs and do the install through installimage on the nvme only anyway
- after it finishes, boot into rescue mode and create a small (~10MB) partition on each 4 TB disk, type set to bios boot (fdisk).
- figure which partition from nvme is your root-partition and mount it somewhere. f.i. /mnt (and the boot partition boot into it, if seperate, e.g. /mnt/boot)
- bind mount dev/proc/sys into that
- now chroot into that system (chroot /mnt /bin/bash)
- use grub-install to write the boot-loader into each disk (grub-install /dev/sdX).
- run update-grub in the end. reboot, and hopefully it starts correctly
now create your sw-raid from the rest of the 4TBs afterwards with mdadm and mount it, adjust fstab, etc.