I think when playing PzC you need to keep in mind that much of what you see is somewhat abstract to real life and hence there are occurrences when units behave in what might seem a odd way, the very fact the game uses a You go/ I go system where both sides cannot more and fire simultaneously will always throw up situations that don't strictly follow historical lines.
In your example the AT guns would have to their own bespoke opportunity fire mechanics that increased the likelihood that they do fire on your opponents turn because otherwise imagine your frustration when the tanks advanced but didn't trigger any opportunity fire which does happen from time to time, players would want to try to disrupt the tank unit on their turn with the hope that the disruption was carried over to the opponents turn.
This idea might be better suited to the phased turn system where players control the units fire on both parts of the turn?
Anyway very pleased to hear you are enjoying your latest round of PBEM games, good luck!