Fill up the lav and then pull the stopper . Does the water drain out with a suction? Or does it bounce the water in the toilet some?
Might pe a partially plugged section of the sewer, or something in the vent pipe.
You say the toilet flushes, but I eonder if it flushes like it should.
Run water with a garden hose down the stack on the roof . You should be able to hear the water draining all the way down the line.
If your on the roof have someone flush the toilet. If you hear a glub-glub, then the sewer is slow and needs to be cleaned to the septic tank.