Idea puke time:
If I could mess with the flaks, I'd give them both explosive direct damage and shattering AoE damage, make them medium-long range weapons with much less jitter (if any), give them an arming time that makes them useless at short range, make their clips 3 to 4 times larger, and then balance damage as needed. Basically, I'd like to see them in a long-range support role (engine/gun disablement and hull destruction).
This of course would create overlap with the Mercury and Artemis. So, I would make the Mercury either piercing only or piercing with a very very small shatter radius. As for the Artemis . . . idk, maybe we could do something whacky with it, like a wire-guided function where you have to turn the gun to guide the missile to its target (tech was developed for this by the germans in WWII, but it's probably just a little too high tech lol)
Make both mortars explosive only (direct and AoE), eliminate their arming time, add a touch of jitter to them, and cut their range down some, so they're limited to melee-short range engagements.
Just throwing things on the table to make flaks more interesting *shrugs*