@mafco77, et al
I have received a lot of messages regarding this, so I figured I would post an update here.
I have not gotten any further with the Stratadue project.
The limitation remains determining how to control the steppers and servo motors through the Attiny85. Perhaps the Attiny85 was a bad choice in chip as I am using literally all the pins for IO and that means reprogramming can only be done via a high voltage programmer. If I were to continue, I would need to swap to a device with more pins or an Attiny85 in a DIP socket.
I cannot remember, but I believe the device requires a Duet2 with custom firmware. Not sure if my changes for the multiple expansion chips was ever merged into the main code or not. It's unfortunate that Duet3 came out soon after I started working on this and it doesn't appear to be compatible either. Bad timing unfortunately!
I do have a few PCBs built, but without the Attiny85 code, and custom Duet firmware, I'm not sure it would be of much use. I still have ~10-12 uPrint printers sitting around gathering dust too! I would love to get them up and running but I believe technology has advanced so much that newer / cheaper printers are leaps and bounds better.