Well, I would start by booting from the GRUB command line. Using the install media can work to fix the system, but this is getting into chroot territory and there is no reason to believe the system isn’t working. Just not booting. Much simpler to use the grub rescue terminal to force a boot, then run all of the grub goodness from there. Basically, if it boots from the disk in rescue it guarantees grub/efi is mounted where it needs to be, from there grub-install on its own should just work. Also, make sure the config you are feeding grub-install is set to output a boot option. When in doubt use the default config, it should work fine.
Well, I would start by booting from the GRUB command line. Using the install media can work to fix the system, but this is getting into chroot territory and there is no reason to believe the system isn’t working. Just not booting. Much simpler to use the grub rescue terminal to force a boot, then run all of the grub goodness from there. Basically, if it boots from the disk in rescue it guarantees grub/efi is mounted where it needs to be, from there grub-install on its own should just work. Also, make sure the config you are feeding grub-install is set to output a boot option. When in doubt use the default config, it should work fine.