I tested the current draw on the 5V line using a OS 3.2 DC, and it’s definitely not 300mA. With the ESP8266 (i.e. top layer board) on, the current is about 80mA in AP mode, and about 50mA in WiFi station mode. This is the case either when no solenoid is on, or a solenoid is on. You said you have a friend who has a OS 3.2 DC, that you tested and it draws about 300mA. I am not sure how to explain that, but the one I tested definitely does not have this issue.
One thing you should be careful with is the SMBJ24A TVS diodes — these are for protection against high transient voltages. These are polarized (unlike in the AC-powered controller, which uses non-polarized TVS where the part number ends with CA and not A): the negative of each TVS diode should be connected to each zone port, and the positive should be connected to the 0.2ohm current sensing resistor. I realized that in the schematic I used a non-polarized TVS diode symbol, so it probably is confusing which direction it is. If the polarity is wrong, when a solenoid is connected, it’s basically immediately connecting the solenoid to ground, thus it will start to draw current even when the zone is off. That said, it still doesn’t quite explain the high current draw on the 5V line, because in holding state, the solenoid draws current from the input power supply, not from the 5V line, so even if the TVS diode polarity is wrong, it still doesn’t explain the issue.