Best of luck. You might have to wait years for a positive result. The power company would have to spend time and money tracing the source(s), politely ask the owner(s) to modify or stop using their noisy equipment, then take them to court if they refused or cut off their supply.
Poorly designed or badly maintained industrial machinery can "chew holes" in local mains. Smaller items such as air conditioners and freezers can generate interference. Expecting an energy supplier to fulfill the terms of their contract 100% is wishful thinking.
There is also the question of the electric wiring inside the block of flats and the provision of an adequate safety earth, especially if the OP is living on the 12th floor. If either of these is deficient, I might require remedial action by the building's owner and partial rewiring. It's not going to happen overnight, if ever.
There are times when you have to take matters in hand yourself, and find a practical solution that alleviates the problem. Fitting a few €12 filters is at least a starting point. It might have zero effect, or it might provide a partial cure. No knowing until you try and much quicker than waiting for other people.
Question. Do you get the same problem with all games on the PS5, or just a few, or just one. Moving to your grandparent's house has made a difference (possibly due to a less noisy mains supply) but you still have a few problems, which might just be down to bugs in a specific game.
Vaguely possible, but very difficult to tell without returning the PS5 to the manufacturer or a skilled technician used to repairing PS5s. Power supplies are failrly tolerant of mains borne noise without dying. Better PSUs include an MOV (Metal Oxide Varistor) which clamps short duration high voltage spikes. Hit them with enough spikes and the MOV can eventually degrade or fail. You also find them in some surge arrester mains socket strips.
https://en.wikipedia.org/wiki/Varistor
The bad news is that if your PSU needs repairing, it's a job for a qualified technician, not for inexperienced/incompetent foolish people. Do not be tempted to open any mains PSU. Some members on other forums issue dire warnings that computer PSUs retain a lethal charge, years after switching them off. More experienced design engineers and repair technicians know bulk electrolytic capacitors on the HT side in a home computer PSU usually discharge below the SELV limit in (typically) less than 30 minutes. In any case, they have job experience and avoid touching the high voltage circuits, before using a device to safely discharge any remaining charge on the bulk electrolytic.
If ATX and PS5 PSUs were 100% lethal when opened, you'd never see reviews on TechPowerUp, Tom's Hardware, etc., where the reviewer unsolders the 400V bulk capacitor from the PCB to get a clearer view and take photographs inside the chassis.
I get the impression there's a significant difference between "stutters" and whatever makes your game "unplayable". By stutters I assume you mean very short duration pauses in the game. By unplayable I assume you mean the game locks up and freezes or crashes completely. Not being a PS5 owner, can I ask if the game is easily
copied to another PS5, to see if it's buggy game software or a PS5 hardware fault.
Do you store games in an SSD inside the PS5. If so, the SSD might be faulty and in need of replacement. Presumably there are ways to check the SSD for errors, either inside the PS5, or outside when attached to a test system to perform S.M.A.R.T. checks.
Were you playing a game stored on your friend's PC? If so, it's not really a valid test. All it shows is the game (same game as on your PS5 or on your computer?) works OK on his system. He might have a faster computer with a better GPU. He might have a more stable mains supply. He probably has an SSD with no errors. The list of differences goes on.
An acid test would be to transport your computer over to his house/flat, but that still wouldn't test your router. You've presumably tested your PS5 at his place?
