Good news first: I managed to get one motor moving with a simple Arduino sketch.
I’m starting to hate QFN packages with a passion. I spent a whole afternoon trying to re-work two pesky drivers. The chips would not communicate via the UART port, no matter what I tried. Finally traced the issue to a bad solder joint on the QFN package and boy these are hard to spot. Simply re-heating the drivers and re-positioning was not enough, I had to remove the chip, add more solder paste, melt it, then re-insert the chip, wipe the excess solder with a soldering iron and finally re-flow the chip one more time. Complete and total PITA.
To top it off this destroys any near by plastic connectors, so now I have the drivers in-place but have to re-solder the connectors back. This would be an endeavor for the next week.
Now all 5 steppers are communicating with the MCU reliably. I had to add support for half-duplex mode to the stm32duino core. The proposed changes are still pending, but I verified that the TMCStepper library is able to communicate with the drivers.