When flushing my toilet the water comes all the way to the rim, sometimes it sits then drains slowly, sometimes I have to plunger it or sometimes it flushes ok but then the water goes way down to the siphon part of the toilet. I also have a shower in same bathroom that has started backing up and then slowly drains out. But the sink in same bathroom drains fine but all drains gurgle. I have a septic system 1500 gal tank emptied in 2016 had five people living here for 2 1/2 years only two now for a year. Looked in my septic tank and when toilet flushed or ran water I could barely hear a trickle coming from house. Stack, clog or main pipe clog?
I would be running a snake down the main line from the toilet, depending on the plumbing the sink could be downstream from the plug. Normally the toilet location is where the vent connects so if it backs up the vent line can be flooded . Running from the toilet is usually the easiest access and only cost a wax seal. A plunger knocking it loose sounds like a better case for a big snake so you can pull the clog out instead of have it move it around.
Gurgling sounds like the air vent might be clogged. That would make the drains move slowly, but not that slow. It might be time for a cleanout of the main drain pipe. Also have you been treating your septic tank? Mom & Dad's backed up after years and it was actually due to powder laundry detergent plugging the main drain leading to the septic tank, but because they had been putting the enzymes in it, the tank was clean. Once they broke the laundry detergent clog loose, the house drained properly.