@dc42 - The issue with this particular capacitor is that one end has a hard stop but the other end does not. At the other end, the cap shaft feels as if it has come lose from sort of internal coupling. Not a good feeling and since it is a Russian made capacitor, I don't have a manufacturer who can give me more info about the cap internal mechanics.
I was proposing that you drive the motor to the end with the hard stop to establish an end point, then count stepper pulses from there to track the position after that. This will allow you to avoid turning the shaft too far the other way, provided that your mechanism for counting stepper pulses is reliable, and that the stepper always moves when pulsed if it is not at the end stop.
I suppose there is nothing you can see through the glass envelope when the cap is near the other end of its travel, that would allow you to provide optical detection of appropaching the other end point?