Plumbing – Rheem Hybrid Water Heater Dead

20 years old, it's almost certainly time for a new one unless it's of very unusual construction - like stainless steel.

If water:

overflows from the top of the pipe onto the floor

Then the overflow pipe is not correctly routed - it should go DIRECTLY down to the floor. I can see it going sideways in the picture on your other question. It also appears to be reduced in size, which is a code violation, and if it's positioned so that water is held (trapped) in it, that's a code violation.

Many hot water heaters need an expansion tank to avoid over-pressurizing the PRV when heating a slug of cold water when no water is being used. Most PRVs are rather bad about resealing when they have been "exercised" a bit. Both of these may play into your problem.

If the water heater has never been flushed and is rather old, there could be severe deposits built up inside the tank at the bottom, partly insulating the bottom sidewall where the lower thermostat is meant to sense the temperature. You might be able to compensate for this somewhat by turning down the lower thermostat to a much lower setting. "Popping" noises are common when a water heater is full of mineral deposits and corrosion from a spent sacrificial anode. Look for the UL label that shows the unit's date of manufacture. If it is 10+ years old, then it is very likely at its End of Life. It is doubtful replacing the heating elements will make much difference.