SPAAG (AA Tank) is super difficult to use

First I need to hold down the left mouse button to 'charge up' before it can fire, then there's a very long gap right after a 6 rounds burst.

Are you sure this isn't a bug? This SPAAG mechanism is so counter-intuitive. 

The damage against infantry is also next to nothing. Couldn't kill anyone with it because of its short burst and incredibly long cooldown time between each burst.

Stationary AA dual cannon works perfection fine though. It's working as how you expect it to work.