Your analysis on artillery is excellent, I´ve run in the same trouble while making my new scenario with the Bombur mod. I would add that the AI have a big trouble in figuring how to use artillery, they use artillery as if they were regular infantry, making frontal attacks and taking huge losses in return. I also considered the fix you created, but I see a trouble, if you make an artillery bombardment, then you cannot move your unit, so it´s, maybe, even more difficult t make an advance. This could be fixed again by removing the ability to make bombardment, artillery would be then used as if they were infantry guns, but more powerful. I think this could be used for strategic scenarios (Bombur mod has a 50km hex and 15 day turn), and it already removes the AI weakness, but I´m not ready to implement such a big design change.
Bombur, yes this has been something that I have been thinking about since the beginning, and realizing that the hexes were really too big to use artillery in the way the game is set up (realistically at least). I started with the thought that one could separate out the artillery from each formation, and still give them a range of two hexes as a design decision. But, in a test game where I had an artillery regiment for each division, it was a disaster. The artillery set up behind the lines clogged the breakthrough and made everything a mess.
A solution would be to "shrink" our concept of the SFT and combine the artillery for several divisions into a single unit. Conceptually, this would like putting all the artillery from the infantry in a corps into a "corps" artillery asset. This could work, but makes a historical scenario at the division level rather messy.
I looked at DC-3 and the artillery in embedded in the units. They seem to have a range of two hexes, though, which is absurdly long for such that hex size. But, to do this, artillery would have to be support unit, like a super infantry gun. Which kind of begs the question why have the two of them? Most definitely, the infantry guns should not have an artillery range since these are typically smaller guns with ranges typically at most 5000 m (for example the 15 cm sIG 33). So, artillery would give you a unit you could use in a static line situation, where you don't want to attack, but want to pound the guy across the way to reduce readiness. But, I suspect, and haven't checked, that this might affect the unit's entrenchment. The main problem is that the concept of reducing readiness prior to an assault can then only be done with air units. I don't know how this would work in the game just yet, and might be unworkable with the current engine.
The scale at which artillery in the game would work reasonably well is 4-5 mi/hex, which each formation representing a regiment, much like DC-1 and DC-2.
With larger hexes, like the GD1938, artillery is even more vexing. Perhaps one way is to just give the artillery a range of one hex. Then, it must be up front, sitting on the unit.
I think this needs to be experimented on. I am in the midst of trying to make a division-level scenario, which is how I ran into the problem of artillery for each division, but I suppose I will have to experiment some first.