It slightly shifts despite the tight coilover spring perch!
If the belt drive is in good condition, then the pump is clearly the cause. I suspect that the injection timing adjustment is simply designed in a way that it can jam. This wouldn't be surprising given the mileage.
A new pump might not be worth it considering the mileage. With a bit of courage and some DIY experience, you might be able to disassemble the part yourself, as described in some technical articles.
It seems to be the fuel injector adjuster. While the actuator diagnosis is running, the N108 valve can be heard adjusting in idle, and the duty cycle in measuring block 4 changes nicely from 4.8 to 94.4%! However, when viewed with VAG-Scope, the values only move between approximately 80-200! Shouldn't they be 'normally' between 0-255?! Therefore, the piston of the fuel injector adjuster is stuck.
Currently, it is unfortunately impossible to find a used, inexpensive pump...