Thank you for such a detailed answer,
I have ordered a replacement driver.
The results were interesting: I followed your instructions and got expected results all the way through my DIP switches Everything was like it should be.
I went back to repeat the process, everything was halved. I had 4000 steps to 20 mm motion, now I have 2000 steps per 20mm. And all the other configurations match that now, so it seems to me the entire driver is having issues…