In our old house that is now used as a farm office it had a partial basement and a floor drain that drained out into a pasture. It got to backing up. The old drain line was hand dug and clay tile placed end to end with no seals. Over the many years tree and grass roots had penetrated between the tiles and filled the line with roots. Took a Rooter Rooter guy a fair amount of time with his drain machine but he eventually got the roots cut out and it has worked ok for about ten years now.

The septic and drain lines Y together some place on the way to the drain field in the pasture.

Don't know your specific situation, but I told you mine so if anything I said rings a bell with you, maybe it will give you ideas to check.