Pull the lid off the septic tank. It might be sitting on top of y'all haven't ran much water. I'd pull that septic tank lid and look on top. Then I'd pull the toilet and check the s trap. If you are on a crawlspace, after checking s trap, get underneath the house and follow the drain line looking for any place it might get hung or settle. You gonna have to remove those pipes or pour a 5 Gallon bucket of hot soapy water down that pipe where the toilet is removed. I'd get a pool net dipper and comb that septic tank after I checked the line and toilet. If you are on a slab, only choice you have is removing the toilet to look and removing the septic tank lid to look around. And pour 5 Gallon bucket of hot soapy water.

If that ring is over 1k bucks, I'd be digging deep. Good luck bro, keep us updated. I hate that for you. Sounds like my luck.