Your first line says “No Dev Mode.” Do you mean Dev Version? In the developer version, the fuel level can only be changed through the EFB. And payload can be changed only through the MCDU. The MSFS fuel and payload UI has been disabled. See Fuel and Weight - FlyByWire Simulations Documentation
Yes, it is specific to the FBW A32NX add-on. You can change fuel mid-flight. You just need to use the EFB to do it. (You also need to use the EFB to change the fuel level on the ground.) On the ground, you have the option of instant, fast, or real refueling time on the EFB refueling page. In mid-flight, it will only be instant.
Setting or changing your payload is similar, except that it is done through the MCDU. The loading speed for payload can be selected in the EFB options.