Here's the solution to self-propelled arty....
When you get where you're going, you set up and it hoists a balloon like this over your point. Your gunner position is actually aimed from the basket, like the 8" guns on the ships are aimed from a control point rather than the turret itself. You can see further to spot where your shells land.
If the trees are some 150 feet (last I recall?) these balloons should go up to 300 feet. Maybe 500 feet? Enough to see where shells are landing some 8-10 miles away (not sure on realistic ranges, fill in with whatever value is right).
This does a number of things to keep these possible arty setups ballanced.
1) The balloon is highly visible. GVs can spot it and drive towards it, shooting the GV at the base, planes can fly over it and spot it easily. It's a nice "look here!" sign.
2) The gunner control position in the basket simulates the radio headset used by the spotters to change the firing coordinates. That in conjunction with the Land Mode firing system (angle, range, etc) will let you walk rounds in as long as you're unharrassed.
3) The arty would be mostly soft-skinned and easy enough to strafe/kill. Even if the gun itself is made of metal the crew manning it is not. This is a stand-off bombardment idea, meant to park miles from a town. That is your protection. That is your safety. It isn't meant to take out tanks (although we've seen shore batteries and ship guns can do that with a lot of practice) and it won't work so well on in-close targets because flatter trajectories will be harder to aim over obstacles (whereas longer ranges go up and lob downward on the target, avoiding small hills and trees, etc). Because your perspective is so high it will also be hard to aim short ranges. This keeps it honest, prevents some of the possible abuses in the MA of the system.
4) Rounds are limited, and there's still a reload time and a "fire, adjust, fire" preliminary ranging to help you get a mental idea of how far the target is from your current viewpoint. You would have to get a teammate to give you supplies
5) You'd have to stop, park, turn engine off, and something like opening doors (see PT boat, open the bomb bay doors just deploys rocket launchers) to send up the balloon. Until balloon is up you can't jump to the gunner position. Nothing overly long, just maybe a 5-10 second animation.
Done this way you get arty deployments, you get the longer ranges, but still the gameplay balance of being stationary, being vulnerable, yet giving that hope and potential to be very effective. Overall a good balance IMO, with the way I've described my idea here.