My walls here are cinder block so not worried about damaging them like drywall.
I managed to get the trap off the sink and set it aside
Discussion
Nice! Was it the source of the blockage?
Negative. Was only small debris in the trap. Gotta be the horizontal pipe or more likely below.
What I do know is that draining the kitchen sink above causes water to back up in the basement sink.
Ooo that’s a good insight. And less accessible on one’s own. The basement sink filling is a clue that the blockage is in a pipe that is common to both of them. Have you tried a drain snake? I’d use it from the bottom sink
I don't have a drain snake. That may be worth buying just to have on hand
Don't think I can get past the 90 degree turns here though. I kinda want that horizontal run replaced with PVC. While I doubt it's the ultimate cause of the problem I would not be surprised if it's severely corroded inside leaving little drainage
It seems like we’ve gotten past the easier stuff.
I think I’d try the snake still, it should make it past the 90 degree bend, but also exactly how much you want to or have time to try before letting a pro handle it is an individual thing so I respect that.
I've got an appointment with a plumbing company coming tomorrow morning.