Wheel Alignment After Lowering Suspension

I've already tried using the search function quite a bit, perhaps not enough.
But somehow, I've been searching for an answer for a long time.
Therefore, I'll simply ask the question:

Who here has actually had their car lowered (I'm talking about...)?
(a modification to a 20mm lower suspension system to make the car safer).
Then have the alignment adjusted (I'm talking about doing this 5-8 times).

and the car is still much worse than it was before?

Specifically, it's about a VW Vento TDI. (Sorry, that word had to be included.)
The back of the sofa (which I bought used) had already sunk in a lot, so I had to do something about it.
What can be done? -> Subjunctive clause.
Word from my trusted mechanic:
'I've sold this many times before, so it must be good!'

Said and done -> everything installed; adjusted the suspension for the technical inspection -> instead of 11 cm of ground clearance, there are only 8.4 cm!
-> Reassembled with original springs (the dampers were defective and needed to be replaced).
For verification purposes -> everything is okay.
A few months later -> replace the front rubber control arm bushings!
You probably think it's no big deal.
That's not the case;
Bolts are rusted -> new engine support (front axle).
-> exchanged
-> It works quite well for a while (2 months).
-> The car is driving worse and worse again.
-> Adjust tracking.
-> The car drives very well.
-> 4 months later ->-> The car is driving poorly again (gradually worsening).


I think I'm going to throw that monstrosity away.



Does anyone have any ideas on how I can check the track myself?
I need the exact parameters (angles).
which I can easily measure using simple methods.