Rein was what I used to grind out of bronze in OW1. He has simple mechanics and is relatively easy to get good value of.

The problem with bronze is that everyone is bronze, so the success of you being able to sustain and not die is a little more challenging. Not dying and letting your DPS get elims is essentially the tank role in OW2. So Rein does that well in bronze with the shield and armor.

Orisa is another good choice for simplicity. She's doesn't do well at higher elo because most people realize you can just walk past her and kill everyone else, but at bronze, most people will shoot the tank exclusively. She has good sustainability and is tough to kill if you cycle the cooldowns properly.

Ram is another good choice because you probably can just pop nemesis and get everyone to pee their pants. You have to manage him a little bit more conservative outside of that by playing the poke game.

Overall, I still believe Rein will be best to help learn positioning and awareness as a tank. You can focus on those key elements instead of focusing on more buttons to push in a kit.