This is a bit of the vet suggestions but I would rather skeptical of adding deflection damage to all AT weapons "spaming" T-70 to counter KT should not be a viable tactic.
I guess one could add some deflection to some weapon or someone could add chance of a deflection damage on some weapons as a ratio to penetration/armor.
So it would break down a bit like this:
Critical hit full damage
Hit but no penetration, re-roll for a deflection damage equal to weapon damage*penetration/armor
Ricochet no damage
It would necessarily be for ALL weapons (the deflection damage) just the important ones. Imo lights and generalist mediums wouldn't have any deflection damage for the reasons you listed. Even premium mediums below panther I wouldn't (usf HVAP being an outlier) but at guns, medium TDs (no for su76 or puma) hand at and heavies could all do deflection damage
Also I suggested something like a more refined pen system a ways back
Basicly if it didn't meet a certain % it would deal no damage (normal medium vs panther front for example) but if the pen greatly exceeded the armour value it would do bonus damage (is-2 vs Kuble kinda thing) granted the OHK in the game have been greatly reduced so idk if the latter is really a good direction anymore.