You say the toilet is backing up. It sounds more like a pyramid has built up under the down pipe. You can try a wand type flusher down the toilet (hopefully you have a straight shot down and not the offset piping) to break up the pyramid. Then get the tank near full and use TankDRx to dissolve the buildup
https://tanktechsrx.com/product/tank...ck-tanks-16oz/