Converted MBR to GPT and menu is gone, can't use grub-reboot either

[Chandler]

Hi all, I recently (like a month ago) upgraded one of our computers that
hadn't received any upgrades for like 2 years.  Afterwards, it wouldn't
boot.

I think it was because Grub2 got upgraded and no longer supported MBR?
(it's now version 2.06-3~deb11u5)
I read something along those lines, that I had to be using GPT now.  So
I actually figured out how to boot a live CD (remotely through the IPMI
I might add) and deleted the MBR and added GPT.  It worked, and the
system is booting again.  The kernel's been upgraded to 5.10.0-21-amd64
and update-grub and co. works fine and system boots normally.

Except!  The boot menu is now gone and I actually just noticed now
that I'm trying to run memtest86.  I tried holding Shift while it
tries to boot, and also have tried tapping Esc, but neither of these
bring up the menu.

Then I remembered I could use grub-reboot to boot into memtest86, but
not on this system apparently because when I tried that it told me this:
WARNING: Detected GRUB environment block on diskfilter device
Memory test (memtest86+, serial console 115200) will remain the default
boot entry until manually cleared with:
    grub-editenv /boot/grub/grubenv unset next_entry

I guess, it's because /boot is on a md device?  so I guess I really do
need the menu working again... any idea what happened to it and how to
get it back?  Everything I found on the internet about this seems to be
related to dual boot with windows?  but I'm not dual booting anything...
All the config and menuentry's are still there in /boot/grub/grub.cfg...

Thanks