First of all, the SPAs aren't even SPAs, they are SPGs, intended for direct fire instead of indirect fire like artillery
but let's say the appropriate vehicles to add would not be possible because they are open topped (like M7 priest or Wespe), so you still want to have tanks like the brummbar or KV-2 be the SPAs, well in that case:
why on earth is the shell's trajectory so wacky? even ignoring the bugs where it just seems to suddenly turn back to hit it's target, in normal circumstances, it seems to just go up and suddenly curve downwards to hit it's target like a javelin missile
why not just give them a regular projectile that just has really low muzzle velocity + a little bit of random spread? I mean some people in comp matches seem to use bazookas as indirect fire artillery, so clearly it's possible to just use normal projectiles in such a role, right?
then there is the fact that for some reason the same squad that operates static artillery also operates the SPAs? resulting in them looking out of place in their tanker uniforms, while also for whatever reason having semi automatic rifles and flare guns?
here's what the solution SHOULD have been:
1- delete static artillery and make it a commander ability
2- add mortar squads that can build mortars anywhere on the map (like AT guns), and not at preset positions, or if too lazy to model mortars, just make it so you can build the current artillery guns anywhere and limit their range or make it so it wont allow you to fire into the last enemy sector
3- make the SPAs into direct fire SPGs as they should be (or if you really want to have SPAs, make them multirole SPAs and SPGs at the same time and nerf the AP shell so they don't accidentally become tank destroyers) and make them operable by regular tank crews instead of artillery crews
4- ???
5- Profit!