Just installing an expansion tank is not really the way to go.
As a LMP we check the actual BTU input of the heater and actually water the gas meter to make sure they match up.
We also check the specific gravity and heating value of the local gas supplier.
Throwing in expansion tanks and by passes and PRV’s is the handyman way of guessing.
Again I will strongly suggest you call in a professional as even though your T&P is doing its job you are putting undue thermo stress on this pressure vessel.
A T&P is not designed to be an operating controller.