I like what you’re saying about limiting barrier tanks to one so that they don’t have to be gimped. I think that’s really important.
I don’t like the idea of two separate queues, or having a lot of double off tank games.
I had an idea that’s similar, but simpler: 1-3-2 with one optional off tank taking a Damage slot. Then most games will still be 2-2-2, but the kind that people actually enjoy (one main tank and one off tank). In other games, you may get actual 1-3-2, and that’s fine as well.
Here’s my thread if you want to look and see what you think:
1-3-2: Introduce the “Heavy” Damage role