I thought I'd make a few comments about what I would do differently if I did the project a second time.

• I would either use stronger stepper motors next time, or DC motors that are stronger than these. They were powerful enough to keep it balanced, but not enough to reliably drive around and steer. I do like the precision of the steppers and not needing encoders though.
• Mount the wheels more rigidly, rather than relying on the precision of 3D printed holes. Or at least get a better 3D printer XD
• As mentioned in the first post, I would triple check the PCB design for places where pullup / pulldown resistors are needed (I hate janking things onto PCBs after the fact).
• Spend some time setting up a method of flashing code to the Exen Mini remotely. That would save a looooot of prototyping time in the long run.