I've reading up about the architectures and programming of computers and consoles from the 80s and early 90s lately, and have been itching to do a similar project of my own, but have been kind of floundering on where to get started. The fact that you pulled this off inspires that this sorta thing can be done.
Have you considered doing a series of blog posts going into more detail on each section of the console and your journey in getting each bit working, describing failures and successes both? I think such would be instructional to other people who want to do some similar homebrew computer/console hacking.
I was kind of surprised that your PPU design was frame buffered instead of line buffered, but I suppose I perhaps shouldn't be. I imagine the PPU chips of old were line-buffered because RAM was expensive in the 80s, and it was a good enough interface to control a scanline-based display. In my recent reading about the architecture of 3rd, 4th, and 5th gen consoles, I noticed that the 5th gen systems became fully frame buffered, as memory had become cheap and fast enough in the early-mid 90s. And a frame buffer certainly feels a bit simpler and more intuitive to think with than a scanline buffer.