I had almost the exact same thing happen a couple months back. Septic
was pumped 6 months prior, but it acted like the system was full. It
would dain slowly, seemed fine for the first couple showers in the
morning then would be slow. I tried several different chemicals and
tried snaking through every opening I could get to. Only one I
couldn't snake was the main cleanout, only because it wouldn't open.
Ended up having a plumper with a pressure sprayer come out. He was
able to get the main clean out open and ran the pressure spray out the
main line to the tank. Things are perfect now, but I had a day of
hell trying to clear it myself. You trouble sounds exactly like what
I had. Oil and grease build up on the out flow pipe. Good luck.