Artillery is one of the more interesting unit types to pick - lots of variety and some hidden features that add complexity.
Artillery (and AA and flame tanks) have Rate of Fire that can be less than 100% or more than 100%. If RoF is 10, you get 10 shots for every 10 strength. RoF 11 (most 75mm artillery and Stug 3) you fire 11 times for every 10 strength, RoF 9 (most 150mm guns though I think the standard german 15cm is RoF
and you fire 9 times for every 1 strength. The number of rolls is always rounded down, so your 11 strength Brummbar fires 9 times (rounded down from 9.9), same as the 10 strength one. Typically a heavier gun has lower RoF (for info, it's listed in the equipment files), so it fires fewer shots, but what it hits is more likely to be killed rather than suppressed. Karl Gerat, Gustav and Dora have RoF of 3 or 4, which is why they never suppress anything but consistently knock one or two points off hard targets. Rockets and 10.5cm guns have RoF 10, which keeps things simple - each strength point fires once.
For hard targets (forts and strongpoints) you also have Fortkiller. Paratroopers, Pioneers and assault guns have FortKiller, which gives +5 attack to structures. So your Stug 3 is more effective against forts than you thought. However, fortifications are a type of terrain that provides a base 4 entrenchment and can't be destroyed. For entrenched troops the entrenchment is reduced by 1 point each time you attack, whether it does damage or not (that's each time a unit attacks, not for each roll of the dice).
Then there's ammo. I always go on about ammo. Lots of ammo means you spend less time resupplying and more firing. This is especially important when providing defensive fire. And range, and whether you want towed or self propelled, but there's only so much space and time.
So for destroying forts and strongpoints, you want high hard attack (including the fortkiller +5 if you have it), but for reducing entrenchment you want lots of attacks. Your 21cm guns have good hard attack but low Rate of Fire and lousy ammo, so don't hit so often but are very nasty when they do. This is why many people prefer the 17cm - slightly better RoF and better ammo. A 10.5 will keep firing forever and is very effective against soft targets, and OK up to about T34/43, but no good against heavily armoured targets. The rockets will suppress any soft target once its entrenchment has been reduced to about 1 or 2.
Once you've removed the entrenchment by repeated attacks, and suppressed the troops hiding there, send in the infantry (or better still an overstrength Flammpanzer 2 with its RoF14, so it can attack 15 or 16 times). You should then have one empty hex, so you can move on to the next, and the next and the next......
So the best artillery for Kursk? A couple of 10.5 a couple of 17cm (preferably with attack heroes), some Stugs, some rocket units and maybe a Flammpanzer and some experienced Pioneers, plus a lot of determination.
For flattening structures and suppressing anything heavier that a T43/43, you want something like a 17cm for the hard attack.
