A followup:
Juni or so 2024 I discovered frame broken at the driveside tube that connects bottom bracket to rear axle, and in my case has a slot piece in it to allow inserting a belt.
I got a replacement frame 3-4 months later and assembled everything back together.
The rear outer tyre Schwalbe "Super-Moto-X" has now worn towards one place a little peek green anti leak stuff.
Didn't catch it earlier, but while checking tyres I now did: the wear is now precisely symmetrical.
So it looks like there was something wrong with the frame at purchase time (2017).
At some point I even started to think that it was due to a habit of resisting pedals more with one leg than the other and I started to pay attention to it, with no effect.

A rear tyre lasts about a year, so I have replaced them about 6 times, now would be the 7th, and it's the first that wears normal
Also new, maybe related, the middle still has profile, the sides not, while I keep it at the same pressure as before. It may be a normal thing for wide (62 mm) tyres - I've noticed this at mountain bike tyres from other people, only that due to the whatever was wrong with the old frame upon acquiring the bike, it had been else for me.
And while I type this, I start to wonder, the bike showed a weird instability, already noticed at the first test ride from the dealers shop and back - I had to pay much more attention when riding, and dealer couldn't explain it.
The instable feeling (it was like difficult to ride just straight) suddenly vanished after putting a basket with some 3-4 kilo stuff on top of the rear rack, so I didn't consider it anymore as a problem.
Taking away that stuff again, and the instable feeling returned, proving that this weight on top of the rear rack made the difference.
So as soon as I get the chance, I'll remove it and check.
If it doesn't make a difference, then whatever was wrong with the old frame also caused that instable feeling.