Have you check the septic tank filter?
Had the same problem, but it wasn't the toilet. There is a filter between the holding tank and the leach field that can clog and slow drainage. I had no idea. There was a second access cover buried in the flower beds that I didn't know was there and the filter was under there. Pull it out slowly until the water level falls below the exit and then wash it off with a hose and reinstall.
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