Something like that would work I think. I like the idea of bonuses for advanced FCS - it’s something I did have for a house rule previous additions. From memory
I agree that some modifier vs HEAT rounds would be the way to go
Also, comparing the Large-Caliber Guns from Tw2k 2.2 and the new addition there seems to be a good match between the Pen and damage values respectively. (I have a screenshot but can't work out how to insert it here because I get an error message saying the file is too large ....)
25mm AP is Dam 5
100mm APDS is Dam 8
120mm APFSDS is Dam 10 and Armour -1
125mm APFSDS is Dam 11 and Armour -1
25mm AP has Pen 13 / 13 / 9 / 3 for the 4 range bands
100mm APDS has Pen 70 / 60/ 50 / 30 for the 4 range bands
120mm APFSDS has Pen 110 / 100 / 90 / 70 for the 4 range bands
125mm APFSDS has Pen 100 / 90 / 80 / 60 for the 4 range bands
So at least for the very large calibers the, roughly, each 10 Pen = 1 Damage and APFS / APFSDS etc rounds have their Pen declining as a function of range. These map to the conversion rules on p110 of the referee manual also if a cross check is needed because armour is not a linear relationship in the new edition.
So, if penalties for HV rounds due to range is something you wanted, you could just map it back to the conversion rules (which would be roughly no penalty for the 1st 2 range bands for 120/125mm, and then -1 at 3rd and between -1 and -2 at 4th range band. So a simple approach would be to say 1st 2 range bands no change and you lose the -1 Armour at more than that. 100mm doesn't map as well, but I suppose you could have the same rule with -1 Armour for > half max range.
Having a look at the rules for composite armour in v2.2 (there's also spaced, ERA, but I'm not looking to be comprehensive) - what is basically does is half the Pen value of HE rounds - it's a bit unclear if this applies to HEAT also, because it's not explicitly mentioned ..... What I think might be easy is to say the -1 Armour for HEAT rounds doesn't apply to CA / CSA armour (which is almost equivalent to your post Ursus Maior, or perhaps make it a +2 penalty so that it's +1 overall (because otherwise APFSDS adjusted for range and HEAT adjusted for CA / CSA would bring you back to the same relative effective again because they'd both lose -1 Armour ..... and what would be the point of that !). Then do a sense check to see if it's going to hit the cannot penetrate threshold for having Dam >= 2+ vs Armour)
So ..... in a nutshell. Perhaps:
APFSDS are -1 Pen at >50% max range (last 2 range bands)
HEAT is +2 vs CA / CSA armour (to keep it simple and not differentiate) and perhaps an extra +1 for ERA
So @Ursus Maior - almost exactly what you said !