Hold off a bit and see where Chris is leading you. I'm glad to see thatThis sounds quite complex and I don't want to break something else. Can you provide me a tutorial for that please, in case the others have no better idea?
lsblkdoes, in fact, show your NVMe drive after all. And I'm glad to see that you have tidied up your drive partitions with your Windows and Ubuntu upgrades since last year.
So my analysis about your situation is wrong... partly based on incorrect screenshots. It's a good thing you didn't delete any bootloaders with efibootmgr, don't you think?I posted the wrong screenshots of gparted from my laptop instead of my PC where the problem is.
Your revised gparted screenshots does not show an Ubuntu boot partition (/boot/efi... with the boot,esp flags, as seen on your laptop). So the more likely case now is that the Windows drive is holding your grub bootloader for Ubuntu in its /boot/efi partition, and that is probably as it should be (even though it is not what you wanted). But I'll scratch my head a little more on that... and why you get the grub> error, yet it will boot from BIOS.