Sounds like the blockage might be in the pipe between the toilet and the tank. My rig has two 45⁰ bends in the pipe before it hits the tank.
Maybe try a plunger?
I read of a case where the owner tried everything then used a "Drain Bladder"
https://www.homedepot.com/p/Husky-Me...-111/301879205
That blasted through the blockage.
I guess the next step if that didn't work would be this
https://www.homedepot.com/s/pipe%20b...pe=text&NCNI-5