Are you aware that Apple hardware pre-2015 was much more open than it is today? I agree with your sentiment for modern Macs, and that's why I'm trying to find a suitable OS for switching.
When there's a limited scope of what drivers are required, it should be easier, not harder, to support certain hardware. Being unable to read NVME drives (which have been around since 2011) is no longer acceptable for an OS in 2020.
AFAIK Guix stays very true to the GNU values of ONLY shipping free software, including drivers. A couple years back I had a similar issue where I couldn't get my WiFi card to work because I'd have to recompile the kernel while including nonfree libraries... which is pretty hard to do if you know nothing about Guix!
Maybe I'll try it again in a while. Not sure if it's available for raspberry pi, but it would be great to try it there and eventually switch to it on my laptop (once JJ change WiFi card!)
I used to have a 2011 macbook pro. I couldn't boot USB media at all. It's not a new problem. After my optical drive died, I had to resort to creating my own installation media on a hard disk partition..
Guix rejects non-free software in their repos, so I would be unsurprised to hear that no contributors use Apple hardware.
I agree that what you are trying to accomplish should be doable and even non-trivial. This is definitely a disadvantage of having a more automated installation procedure.
When there's a limited scope of what drivers are required, it should be easier, not harder, to support certain hardware. Being unable to read NVME drives (which have been around since 2011) is no longer acceptable for an OS in 2020.