Other thoughts on artillery:
The main thing it does is de-entrench with 100% chance one level per ammo, once you upgrade at least once.
But more than that, it doesn't even need to be at high readiness. For example, an upgraded artillery which is understrength and not under command of a friendly HQ will still de-entrench 100% of the time. No spare MPP to reinforce your understrength artillery? No problem cause they don't even need to be at full strength!
I believe it is less likely to deal strength damage if it has low readiness, but this often doesn't matter. You can easily rail your artillery around the map and not need a HQ for it to completely de-entrench an enemy unit, leaving it as good as dead. This makes it extremely easy to concentrate artillery against one enemy nation like France of Russia, in order to trigger an early snowball effect (high losses for one nation lead to low national morale and friendly HQ experience gain which leads to even higher losses which are too difficult to replace with the budget of one nation alone).
It seems clear to me that artillery's performance ought to depend largely on its readiness, just like all other units in the game.
I think it's tough to get artillery right, and I don't want to nerf it too hard. German artillery smashed Russian units in Poland in 1915, but even greater concentrations of artillery failed to annihilate German units in 1916. It should be powerful but still have difficulty completely breaking through extensive trench lines. Artillery should actually be LESS devastating against heavily-entrenched enemy units. Ideally it should also lose steam the more it fires during a single turn, to simulate loss of accuracy due to barrels wearing out.
A humble proposal:
Arty De-entrenchment chance by upgrade level:
Level 0: 0%
Level 1: (0.5 - (0.05 * # of times fired this turn)) * readiness %
level 2: (0.65 - (0.05 * # of times fired this turn)) * readiness %
In other words, assuming 100% readiness, level 1 artillery would have a maximum of 50% chance to de-entrench every time it fires, lowering down to a minimum of 5% after firing 9 times. This would yield an expected 2.0 de-entrenchments if it fires 5 times, but only 2.8 if it fires all 10 shells at once.
This would also have interesting strategy effects regarding artillery placement and concentration and shell conservation which I won't go into here. But I will mention that it would also lead to more sporadic artillery fire (checking to see if you get the de-entrenchment you want, calling of an attack if you get unlucky). In a way that might be more "realistic" with units coming under moderate artillery bombardment often rather than artillery only showing its face during massive assaults.
I'm not sure it's the ideal solution. Concentrating multiple batteries together might still be too strong. But I wanted to put my idea out there.