This sounds like a problem I and others have been having with ubuntu and grub for quite awhile. The “initramfs unpacking” error I don’t think is involved, I get that and I can still get to the system. The problem is that ubuntu seems to be breaking grub in some manner with each update, it’s happened so many times, I think I have several bug reports, latest one #1894242.
I have a number of distros installed, and if somehow the grub “updates” slip by, I’m only left with usually U-MATE as the auto-boot. The chroot idea work awhile back, but only by chrooting into an Open SUSE system and then running their version of update grub does it work. I have to use Super Grub 2 disk now to boot that system . . . . PITA
PS: Point being that running “update-grub” in ubuntu doesn’t seem to fix the problem with grub . . . devs don’t seem to be able to diagnose it or think it matters. I think it has something to do with EFI boot, but I don’t know how to address the problem that seems to be happening over and over . . . . OpenSUSE doesn’t seem to have this problem and I don’t think Manjaro does either.