It is rare that your ship will be firing at multiple targets at the same time. Fire at Will simply gives your turrets "permission" to fire on your target (s) when they have a clear shot. 9 times out of 10, they will be firing at your primary target, since you rarely have more than a "single" primary target to deal with in any given combat encounter anyway.
Not sure about that.
I'm only ranked "Dangerous" and the vast majority, if not all, the "big fish" in RESs come in wings and my turrets happily chew on all of them at the same time.
The "trick" to smart turret use, IMO, is in fire-group allocation.
I set all my bigs ships up in a similar way:-
1) Manually-aimed laser.
2) Bunch of turreted lasers.
3) manually-aimed kinetic weapon.
4) Bunch of turreted kinetic weapons.
Then create 5 fire-groups which reflect those 4 categories
and an "all weapons" fire-group as well.
That way you can always "disable" your turrets, when required, by selecting FG 1 or 3, you won't waste ammo by allowing your kinetic turrets to fire inappropriately
and you'll have a "fire & brimstone" setting for those SHTF moments.
If that sounds like too much of a faff, at least ensure you put turrets in their own fire-group.
ALWAYS have a way of shutting down your turrets by switching FG.
Never, ever,
ever configure a ship so there's no simple way to disable a turret. That way lies friendly-fire bounties.