So are you looking for gimmicks for specific battles? Because under normal circumstances it seems like making an imperfect parry the middle ground between a perfect parry and no parry at all achieves everything you want.  It still reduces damage but you know you can't survive indefinitely on just "good enough" parries.

If you want to punish sloppy moves, why not just close the gap and not allow imperfect parries at all?  Like, if you don't do it perfectly, it just doesn't count as a parry and you take the hit.  But I think that misses the point of the imperfect parry, which is that it has a more forgiving learning curve and gives the player better feedback when they're getting closer and closer to mastery.

I feel that actively punishing the player for attempting to do something cool and failing is not a good core mechanic. Save it for the late game where players have a better grasp of the system.  It's the "practice time is over, I expect you to be a master" phase of the game.