I created an MR for raspi-firmware that takes some of these concerns into account: https://salsa.debian.org/debian/raspi-firmware/-/merge_requests/40 It's mostly intended to make U-Boot's EFI implementation easier to manage, but I assume it would be applicable here too.
The firmware is loaded from the /firmware directory before Linux is started. It's actually loaded by some component of the Raspberry Pi firmware before even the bootloader / UEFI binary I believe, since U-Boot somehow has the needed device trees already in memory when it goes to boot an OS. I have been able to get a GUI on a U-Boot EFI + GRUB-booted Debian Forky installation on an RPi 4. -- Aaron
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