I appreciate the blog post and the writeup, it might be nice to include it in the repo.
I have been toying with a similar design, with many of the same choices. Although for the system controller pair, I'd go with RP2350B and ESP32-C61 (I think). It would be nice if there was an optional chip and pad layout to support legacy or classic BT.
Another option would be to have a USB port and support something like https://www.8bitdo.com/usb-wireless-adapter-2/ to enable legacy controllers.
It looks like supporting legacy BT while noble, could be a project killing sidequest (if you didn't already have it done!) Another out, would be exposing an SPI connection internally so someone could hack in a controller of their choice.