It’s a kitchen drain but not like in the US. Garbage disposals are banned here. So there would be no way for wipes, qtips, or anything big to enter the drain. It’s a terraced house in a dense city, so no trees, which likely rules out roots.

The city water is /very/ hard, and past residents likely put plenty of oil down the drain. Every time the drain regurgitates something, it’s a stinky white substance that looks like it has coffee grounds. I know not to put oil or coffee down the drain but past residents are another story. So I think a mass of fat, coffee, and minerals from the hard water could be culprits.

WRT using a pressure washer, I think they would be an option if there were a cleanout with a straight shot. That youtuber would probably be paralyzed when coming into a ridiculous series of tight 90s and no cleanout or vent, and possibly goffered pipes.