Flaks already have air attack priority as far as I know, if you press "Stop" flak truck will first target the nearby air units and only switch to ground targets when there are no aircrafts.
The problem is. When you rush behind Hinds with flaktrucks its a no go to press S to Autotarget the Airunits because they outrun the Flaks.
I would wish its another key like F for the Flaks so they wouldn't stop at a moment were you normally want to chase them.
Or a pure Antiair mode