Sorry if OT, but wanted to agree that a "pseudo ground" floating in the air is indeed useless. The best if the electrical wiring does not support it really a true ground is not just "a large mass of conductive material", but really, the one such object we all share in common aka Earth.
The conductive mass being the water table, or at least your thick copper rod extended far enough down so its bottom is in ground that is wet.
A wire from the end of that goes back to your flat, and that then becomes your "common" also for connecting to metal chassis's, and the negative return of your DC circuits.
Having different devices and circuits tied into "common" at different reference points, that is not true Earth ground, can actually be dangerous, without taking precautions like GFCI / AFCI / GFI etc
These terms and associated wiring "standards" & codes vary by location and jurisdiction, with any mains power issues, best to get in a professional.
Thanks, unfortunately the water pipes are PVC.
Someday when I got a vacancy I will connect the strip ground to a alu window for testing.