I think I’m really hooped now - I disabled secure boot, booted into my install usb. Mounted my system and efi partition. I ran manjaro-chroot /mnt and attempted to reinstall grub like so:
grub-install --target=x86_64-efi --efi-directory=/boot/efi --bootloader-id=Manjaro --recheck --disable-shim-lock and grub-mkconfig -o /boot/grub/grub.cfg. Reboot and same thing: grub_is_using_legacy_shim_lock_protocol not found.