I had an intermittent slow turnover k24, the starter bench tested visibly ok.
It would generally start with a jump, although the battery was fine, sometimes it wouldn't start with a jump, ten it would.
I stripped down the starter and it was chocka with brush material. There was a bit of damage on the comutator which must have been eating the brushes.
Anyway TLDR, double check your starter motor.
/ how easy does the engine bar over by hand?