The gameplay of self-propelled Tanks is to Flank Tanks where the Enemy does not expect it, but there are few places on the maps where you can Flank, I suggest increasing the Area like the Partisans, after that the self-propelled guns will become useful.
I will even suggest make this for all tanks… it will make them be more aggressive then the current campers 
You should be able to use armor to smash the enemy lines while your guys take the objective…
Such tactic is mainly used by US which has fast and turreted tank destroyer.
Tank destroyer from other nation won’t really become useful because of this.
