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Sorry, misread and conflated the early posts where a Sesto was mentioned!
I am not familiar with NINA's focus routine, but the only time I had sawtooth HR readings happening it was wrong backlash setup or other looseness in the system.
Having just re read your first post, Is the focus cube properly attached to the shaft? being able to move the focuser two millimeters by the other focus knob before the mechanics of the focus cube stops it does not sound at all right. On my refractor you cant move the focuser manually. The only movement of any sort is the small amount of springiness introduced by the flexible coupling where the focus motor attaches to the shaft. If the cube is attached to the 1:1 shaft directly (Assuming it has a 10:1 or similar knob on one side) and you can move the focuser a couple of mm by the other knob there is likely another issue there rather than backlash in the focuser itself.
Have you got any photos of how the cube is attached?
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