Currently, SPAs(self propelled artillery) are inadequate at engaging in direct combat, while it is possible to manually change the gun's elevation using Page Up and Page Down(and use the shift key to do these slowly), you can't know where the cannon is actually pointing at since there are no crosshairs and the view is independent from the elevation.
Even if you know where the barrel is pointing at, you can't know where the shell will drop, especially since there are(I think) different amounts of charge(I think they're set in the artillery computer[close,medium,far,extreme ranges..])
My suggestion is adding an action that lets you toggle between direct fire and indirect fire(current situation), or just always being on direct fire, and indirect fire can only be done on the ballistic computer.
In direct fire mode, the HUD should change to a tank's hud instead of artillery/mortar HUD, the barrel should point at what your targeting reticle/crosshair is pointing at.
Shells should be shot at maximum propellant/power, since you obviously need penetration.
And you should be able to zero your turret like in direct fire tank turrets, at increments of 100 m.
As for balancing: The SPAs are large enough already which would make them a high profile target, their chassis should be equivalent to their tank counterparts(Scorcher-Slammer, Socohr-T100), however the actual turret should be very vulnerable since it's very boxy, and probably thin to allow for storing ammo.
I'm not sure if they should have thermal sights, I haven't found whether they do in real life, and for the sake of balance they shouldn't have thermal sights in the game, otherwise they would pretty much dominate tanks with their GMG, HMG, direct and indirect fire.
ALTERNATIVE IDEA(simpler):
The HUD will show you the required elevation to hit what you're aiming at, but it calculates a direct fire trajectory instead. For this reason, it won't have a minimum range for the calculation. This method makes more sense because whatever you're targeting with the HUD is in your direct line of sight, so calculating an indirect fire trajectory(like now), is redundant and makes no sense. {F21935}