Thanks to a hint from another XS-triple rider, I found out that the German owners club actually does replacement ignition advancer springs. Now ever since, the ignition had been wandering quite a bit at idle and when hot I could also hear pinging at mid-rpm.
New springs installed, old black springs next to the ignition baseplate. Note: you have to cut them on one side a bit or otherwise you can't hook them into the holes on the flyweights.
Checking up on the eyeball-timing I did at my mate's place and as you can see, the timing is so stable now, you can even take photos of it.
And as I had the tools out anyway, I bent the bulb holder of the taillight a bit tighter, so the rear light would stop flopping around.
Only downside to all of this: Now I really gotta tackle the carbs again. BUT I have an absolutely solid idle now.