Let me reinstate the importance of the PI and not any other ARM platform: the VideoCore IV documentation is still (somehow) available online. It's probably the only platform where you'd be able to code "down to the metal" the graphics stack in a good old amiga style. We even have a (beta quality?) vbcc port for the VideoCore IV ...
Just to say ... I jointed, albeit with a bit of a low profile for now, the Patreon campaign.