"efibootmgr" doesn't always work with Macs and IIRC the author of it mentions that it shouldn't be used on Macs. Both of the threads you linked to are created by Mac users. Here is something else you may attempt:
- Install Garuda Linux, while installing skip bootloader installation but create a FAT32 ESP and mount it to /boot/efi.
- Chroot to your install.
- Run these commands:
grub-install --no-nvram --removable
grub-mkconfig -o /boot/grub/grub.cfg
"--no-nvram" skips calling efibootmgr, "--removable" installs GRUB to the default location so the firmware can pick it up.
4. If you set up a MacOS & Linux dual boot, hold "alt" while Mac is booting and choose whatever that is not MacOS. If you nuked MacOS, Linux should boot automatically.