RLD/RLD/RLD Gunners. 

Both will be reload capped but soft supports will have much more repair and much less weight then reload gunners. Huge gunners are a waste of displacement.

Unclassed supports can use every gun up to and including 5" which is pretty much anything you would want on the T slots. 

Just thought i'd point out the obvious, you can cut a significant ammount of crew off your gunners and not see any change in ability due to caps. Soft gunners however require a full crew or its a pointless effort. So yea, I'm not going to figure the math but its a safe assumption that you can have lighter normal gunners then soft gunners at higher levels.

 

I do agree with everything else you pointed out though. 6" AA has no place on a BB anyway and are only semi useful on 2 ships.