Having said all that, the system needs to be one where orbital fire support gives a significant advantage but doesn't completely rule the battlefield. Open to suggestions
I favor the abstraction approach myself as well, but I'm thinking about it in maybe a different way.
As I see it, we have two mechanics, "bombardment" from artillery and "orbital bombardment" from ships, that maybe should be treated as one mechanic. As powerful as they are, they don't win the battle; you still need boots on the ground for that. Well, mostly anyways; I think a fleet with enough firepower and a scorched earth policy should probably be able to glass a planet from orbit without landing troops.
So first off, I think ships should be able to fire at a planet like they kind of can now, but just have the damage randomly (not necessarily evenly) split between population, buildings, and troops. Call that general bombardment.
Then there would be what we're talking about here, maybe call it support bombardment. I think artillery and spaceships should probably work exactly the same there; at least I can't think of a major difference in how they'd probably be used. This also means that the defenders could effectively match the advantage of orbital bombardment by their own artillery units (until they got blown up, anyways). There should still be a chance of collateral damage (as should all ground combat, though maybe support bombardment should be higher), but it shouldn't be the primary damage source like in a general bombardment.
The question then is how to ensure you can't completely destroy your enemy with support bombardment. There's already a module for calling down artillery fire, which is good, but there should probably be some method to limit how much that benefits you. Having a thousand ships providing bombardment shouldn't be a thousand times as effective as having one, basically. One option is to give a bonus to the rolls of the supported unit, but I'm not so sure about that; might be too abstracted for me. I think a good balance mechanism might be to limit how much artillery can fire at a single enemy unit; after all, targeting is the issue, and landing a hundred shells on the same spot is probably not that much more effective than a dozen.
That just leads to another problem in that not all artillery is created equally, though. If your ships have 30cm lasers and 10cm lasers, you probably don't want your limited artillery shots to all be the 10cm ones. The specifics of this might depend heavily on how the balance details of ground units shake out, but one option I can think of is to have the game automatically assign the smallest support artillery certain to kill the target if it hits (or else the largest available if none are certain kills). That would probably result in a fairly ideal spread most of the time.