Toilet bowl fills almost completely before flushing
To me this sounds like a partially plugged toilet. A toilet auger will probably clear the problem up unless the drain has been sanded. A toilet auger is a short heavy duty snake made to clear the U bend built into the toilet. I have seen bad renters put sand in the drains of a home this makes clearing the blockage tough as the sand keeps falling into the U bend or trap, but it sounds like a blockage close to the toilet is your problem, once the pressure or water level is high enough the natural siphon takes over and clears the bowl.