[john70t]

I'd think only a slow draining would result from venting issues.
The active backing up sounds more like a main unless the roof vent is vacuum sealed.

Any trees growing closer than a hundred feet to an unsealed main will sneak 10-20ft of roots into that nice continuous flow of nutrient water.
They can squeeze through 1/4 inch.

You can rent cutting machines to bust them up.
There are different tips for the machines.
I think he used the oval all-purpose one. Slow gentle speed and a lot of pushing at first.

But if the problem is the main, why isn't it first coming out of the shower first? (3 feet lower than the sink)

FWIW: I had a basement backup and used a 50ft snake with no results. Called rotorooter. The guy found the problem at the last 10 feet of pipe next to the city hookup connection. He had experience with the cutting machine and could feel when it was binding and where. Back and forth and yanking back. He had technique.
Got a video camera inspection done as well. Total was about $500 or so. It turned out there was a small bubble in the junk Orangeburg pipe just big enough to collect the two rolls of daily toilet paper.