If your relief valve is rated around 70 psig, then IMO I would keep it at that. If a manufacturer supplied the water heater that way then it is best to follow the manufacturer’s design. The relief valve may not have a temperature sensor on it so there is no security about a runaway overheat condition. If there is no temperature probe on the existing relief valve, you could add a long T&P valve into the tank opening with a 3/4-inch street tee and then add the old 70 psig pressure relief in the branch.
You can add a domestic water diaphragm expansion tank to the water heater to insure that there will be air in the bladder to take up the expansion. The expansion tank for domestic water has an inner coating to prevent oxygen from rusting a hole inside the tank within a year. In this manner, the tank expansion remains independent of the air quantity in the water.