If your power supply is rated 24VAC, 1500mA, it’s more than sufficient to drive the OSPI, RPi and solenoid valves. If you find that the voltage drops to 6.5VAC when the solenoid opens, you should check if the solenoid may have a shorting somewhere. That’s the only reason I can think of that causes the output voltage to drop that much. Measure the resistance between the two wires of the solenoid — typical 24VAC solenoids should have resistance between 20 to 50ohm.