Well, part of balance is how good the class is at doing its thing. This class' thing is "taking the hits." And two of the archetypes aren't actually capable of doing that. So I feel that's a pretty big issue that needs to be addressed.

You're not "taking the hits" unless things are hitting you. And you're presenting your enemies with a very straightforward decision. They can hit you and lose, or they can hit your allies and win. They're not going to take the option that makes them lose, so they're not going to hit you.

You need to start by making a base class that can do its thing. Right now, only the defender gets the kind of class features it needs, so poach those for the main class. The taunt idea is good too.

Other things to consider: How do you physically keep enemies away from your allies if you're not fighting in a 5-foot-wide hallway? How do you discourage ranged attackers from targeting the weakest party members? If your ability to defend your allies relies on reactions, how do you deal with multiple attacks in a single turn?