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that just looks like a backflow preventer, nothing on the heaters side of the system to reduce pressure from the street? if there is no pressure reducing valve on the heater side of the piping, then you need to set the pressure to roughly your street pressure, and either way you will want a DHW rated expansion tank

Hold on. You’re missing something here!

The pressure in the tank is supposed to be set to match the minimum pressure in the system. In other words, the system pressure determines the tank pressure, not vice versa. Therefore, there’s no fixed (one size fits all) pressure setting.

It makes no sense to set it at 15 psi if the minimum pressure is 60 psi or more. That would greatly reduce the acceptance volume of the tank and render it virtually useless.

It would appear from your description of flushing minerals out of the tank annually that potable water is also being heated by the water heater. Correct? That’s unfortunate because you run the risk of Legionella contamination in the heater from stagnant water from the heating system. However, there are a lot of these types systems out there from days gone by.

At any rate, you need to find the static water pressure on the water heater and set the air pressure on the expansion tank to the same. That must be done with no water pressure on the expansion tank.

Your circulator is also mounted incorrectly. It needs to be rotated to where the motor is horizontal, not vertical.

If this is correct — that is, if that valve is just a backflow preventor and not a pressure reducing valve, then the pressure tank needs to be set to the domestic water pressure when the system is cold, and the pressure tank needs to be certified to operate at that pressure.