Hi @DystopianDissenter, at some point in the past six months or so we started seeing a lot of issues installing onto older Macbooks. Look at all these threads that come up with a simple search:
- Mac Book Pro13" 2017 Could not install bootloader - #4 by fumangy
- Trouble installing from live usb - #2 by BluishHumility
- Okay so EFI is broken or what? - #17 by RockyC
- Will the bootloader problem be addressed - #3 by GusGoose
- The bootloader could not be installed
I'm exactly sure what the deal is (I don't have a Macbook, and some of the threads are hard to follow), but it has something to do with when Calamares calls efibootmgr during the Grub installation process. The issue seems to be specific to Macbooks, and Arch-based distros.
The last thread linked up there has a solution which is both fascinating and bizarre, involving booting to a Debian live environment, installing efibootmgr, manually writing an EFI NVRAM entry for Garuda, then booting into the Garuda live environment to repair Grub from a chroot. *wipes sweat from brow*
As bizarre as that solution is, it did end up working for multiple people. If you are feeling up to a little hacking around, it might be worth a shot.