I did tell you what a few differences are, but telling you those doesn’t help you if you don’t understand WHY Apple designed their Silicon that way. There are several good articles out there that describe how Apple Silicon’s GPU works, how it’s different and why you won’t see PCIe options (again, unless Apple makes big changes, which they’re free to do) that should help you with your “why’s”.
Display output at boot could be tricky, and maybe they'll skip that and say, "nope, you can only use third-party GPUs for non-primary displays". But other than that, I don't see why they can't do that. I'm not saying they will, but they probably could.