Why produce AA to protect your troups when interceptors can do that more effectively because they are faster, this way the AA doesn't slow down your stack.
Infantry? You start with more then enough at the start of the game so why would you produce more? Diversify your army and go straight for 5 AT, there after 5 artillery and then 5 tanks. With those troops you will tear any player that wasted his time producing infantry to shreds.
Subs: they should be the first ship in the list. They are stealth to almost all units, highly effective against unescorted troops, kill expensive ships like cruisers and battleships and they don't even see it coming. Three subs locking a destroyer into combat will sink it rather fast.
Also I don't use a lot of Navy as ships can't conquer provinces. Navy vs navy is just a waste of resources really. Ships are only usefull to protect your troops overseas and to protect your coastal provinces and subs are just fine for that.
Yup, additional infantry while 70% of your starting army is infantry = unneeded.
Depends. If you know your enemy to be heavy on the subs, using naval bombers and destroyers can be a better choice. Means you do not waste resources while your enemy usues quite some metal.
That is VERY true. Lot of players do not get that. There is an exeption though. If your core provinces are on the seashore. Battleships can ruin your cities. Very little you can do about battleships if you do not have your own navy.