I have one bathroom with toilet, shower and sink. The toilet flushes very slowly, water starts to swirl down then stops, leaving solids, paper etc in bowl. If it does flush completely (after multiple attempts) I can hear gurgling in the shower drain and kitchen sink drain. I have had the septic pumped and plunged the toilet with no change. Help!

If it does flush completely (after multiple attempts) I can hear gurgling in the shower drain and kitchen sink drain.
This indicates a blocked vent. Locate the lavatory roof vent and snake it out. Put out enough cable to reach the base and 20 feet more to get to the main. If there's still a problem flushing after you snake the vent Look down at the bottom of the bowl. If there is a small hole, then that is a jet that starts the syphon action. If it's clogged the water will just swirl around and slowly go down leaving solids behind. Take your finger,(UGH! ) and run it around the inside of the opening. Over the years minerals build up and cut down on the syphon (flush) action. If it is rough or you feel build up, take a table knife and put a bend in it to get around the curve in the bowl and chip and scrap it clear. Next take a coathanger and clear out the holes around the rim. They start the swirling action. And last, check the water level in the tank. It should be 1/2" below the top of the over flow tube. And speaking of the overflow tube, Make sure the small 1/8" tube from the ballcock to the white overflow tube is connected so it discharges in it and that it's flowing when the ballcock fills. This is what raises the water level in the bowl. For a good solid flush they all have to work together. Hope this helps . Tom