I don't think there is any reason to up the AC above 2. Armor class is very powerful in the scope of how Fifth Edition works. I felt that if I upped the AC it would make it a no brainer to use the tower shield over a standard shield for all builds. Giving up your action for a huge bonus to AC by way of 3/4 cover was the way to go in my opinion. This is a bit better than using the dodge action during combat.
Movement being halved is because you have an extremely large shield that is hard to lug around a battlefield without it getting in the way. It is also pretty heavy. Losing half your movement isn't a major thing but it certainly can be inconvenient. While I don't think this will turn too many people off from using a tower shield there will be times in game that this could become a problem. For example a player involved in combat away from you may really need some help. If you can't get there it could mean death for that character.
If someone in your party has a reach weapon this is when the tower shield is at its best. If the pc with the tower shield gives up his action to use it for cover another player with reach can also benefit from the 3/4 cover as long as they are right behind that tower shield pc.
So overall I don't think this is too over powered but I also don't think it is too under powered. This is intended to give another option for a shield but not be the only option. Much like the buckler shield I wrote up I think that these could be fun options for people to use in their own games. Obviously anything I suggest on the blog like this you must run by your Dungeon Master first to see if they agree with how these can be used.
As an action can declare 3/4 cover
Party Members can stand directly behind you and use you for 3/4 cover as long as you give up your action as well