Now with all the hardware details worked out, and because I understand the basics of Pico SDK programming. I think it'll be a bit easier this time around.

Differences from last time:

• Using Pico instead of Pi Zero - so boot time is instant
• No support for 5200 or Intellivision.
• I'm not going to worry about cost, but I'm going to try to keep a board to less than $50.
• Limited support for controllers.
  • Those that report as Xbox 360 controllers like the 8bitDo SN30 Pro (and actual Xbox 360 controllers)
  • Modern and Classic Atari VCS controllers (I just ordered one of both)
  • Still going to support Keyboards and Mice - I'll do the basics, but for more advanced applications, I'll leave it to others to expand the Pico code
• USB support for sure, maybe Bluetooth. There are methods on the internet for using Bluetooth with the Pico... but I'm not sure if there is C code for it. It'll be a future addition if it ever gets done.
• I'm adding an SD card option, technically there was one on the Pi Zero. I mostly just want to see if I can do something useful with it. I don't think I'll make it necessary to have an SD card in the reader in order to use the device.
• Definitely planning to power it using only power from the controller ports.
• It will have an LCD attached.

I started wiring the setup last night. Here's a photo showing the chips before I started connecting everything together.