Halve damage, two shells with a long reload between each shell. Anymore and it's simply firing 24/7.
I made the following suggestion a long time ago but it didn't get any traction. It's kind of similar to your proposal.
Change the LeFH, ML-20, and B4 to 320 mp, 12 pop cap
shells:
LeFH - 4 shells, 150 damage at center
ML-20 - 3 shells, 200 damage at center
B4 - 1 shell, 600 damage at center (yes, it wouldn't 1-shot a full-health medium)
Revert the AOE of the howitzers to what they were a year ago (don't remember exact ranges). The current AOE is too small to provide any reliable damage. Give BB4 largest AOE, followed by ML-20, then LeFH.
vet abilities: smoke, artillery flare, -15% cooldown at vet 3 (don't bring back precision strike)
give regular shells suppression (but not enough to insta-pin), smoke would be smoke only
Make it so gun can't be destroyed while weapon is crewed, and keep the restriction on not being able to build in base.
This would greatly cut the number of shells while only slightly cutting pop cap to discourage building too many of them (could also put a limit of 2). It indirectly nerfs the counter (stuka dive bomb, IL2 precision strike) by having it affect a unit that is only half as valuable.
The old ML-20 used to be great for punishing blobbers. The current one is RNG garbage.