So, an R7 Astromech.

I completely disagree with your assessment that canceling all dice is OP, even if it also prevents Gunner/Cluster/Corran retro-fire. It only annoys most fleets, and saves your bacon if you're about to die (for one turn), but is really only a problem against lists that rely on a single ship to do most of their damage. Against a list of, say, 4 X-Wings, it's on par with the Chewbacca crew.

Against those heavy-hitter fleets, suddenly you've taken a full turn of attacks out of them, which brings your list up to their level of tankiness, and helps to counter Fat Han in an unexpected way.

"It only annoys most fleets."

So, you purposefully want an upgrade that is disruptive to the way most people play the game?

I can't find another upgrade that completely cancels an attack, let alone one that cancels an attack and then cancels future attacks for the round.

As an upgrade that could be discarded it could allow the defender to choose which dice the attacker must re roll, then remove a target lock from the defending ship, allowing a reasonable and realistic flavor for chaff and hindering upgrades such as "fire control system" without nullifying them completely.

The R7 astromech you mention requires the defender to have a target lock on the attacking ship, and then spend that target lock, however that is a persistent upgrade card. And discardable ones should have a little more 'oomph' to them.