This is a very over complicated answer for the situation.
Removing Open Que COMPLETELY is simply the best option for the game:
Separating the player base into 4 different ques does not benefit the game AT ALL, removing the Open Que and just having Role Que will likely lower que times for all.
Simply que for DPS and Tank only if this is an issue.
I may have misunderstood this point but even in role que you have a combined rank? So as long as you place you’ll have a combined rank