Speed sensor! Wonder how that would give you a rough feeling through your motor, its not even fixed on your motor! Let us know how it goes.
Took some time but the speed sensor came in and was swapped out. Immediately out of the shop the speedometer wasn't working, flipping to measuring between full speed and 1/2 speed. I returned to them with a video of it and they determined it was likely the magnet was missing a revolution on occasion. While they couldn't reproduce it hanging on the rack they tinkered with it and had me try again and the speedometer issue seemed resolved.
I rode 6km home and the first 2 I almost thought the issue was resolved but unfortunately the last 4km the problem remained: the max speed of 32km/hr is unobtainable on flat ground even in turbo. The cutoff kicks in variably and seemingly aggressively accompanied with a vibration felt through the bike anywhere between 28-30km/hr. To make sure this wasn't all in my head I hopped on my wife's Gazelle immediately when I got home and hers behaves as expected. Smooth power delivery to 32km/hr before a proper cutoff.
I've reached back out to them for next ideas, I could possibly pass on the circlip suggestion again but suspect they will be sensitive to outside advice.
@Stephen - Bosch Team tagging you as suggested by another member if you have any insight it would be appreciated.