I did end up publishing the libraries I wrote for VCS cart dumping [1] which handles the vast majority of custom bank switching schemes, with instructions on how to wire it to a rpi or Arduino to make a ROM dumper if anyone is curious.

Looks like past-me also posted an earlier/incomplete version of the paddle and joystick handler code as well [2]. I should find the finished one on my old laptop and push.

1. https://github.com/drzaiusx11/WiringVCS

2. https://github.com/drzaiusx11/AtariGamepad