I believe Pazuzu identified your problem. I had a similar problem with our cabin down at the lake. Turned out to be the line to the septic tank was full of thin roots. Like a root ball. All it would do would let it trickle of water go through. And of course the more solids that went down the line, the more things got clogged up.
If I didn’t run any water for 24 hours I could get two or three flushes or one shower before things plugged up again.
I went to Home Depot and got a professional grade drain auger with both a 2 inch and a 3 inch cutting blade. Watch a couple of YouTube videos before you do it yourself. At least for me, that solve my problem.
Here is the one I got. You can rent one, but if you have roots in there now, you’ll have roots in there a couple of years from now.
