it will sound like a pretty negative remark, but i think you should fire your "think tankers".
imo, mmr is more a problem than queue time.
moreover, when there is mmr problem, problems comes from unbeatable monsters, not unbetable antag, (wich hardly can't be compensate by monsters. in all cases, even with 0monsters an unbetable guy will still beat your ass).
so you can solve the mmr without touching queue time...
just focus on caping your difficulty high and down, to prevent boring missions or insane picks of difficulty wich is what we are talking about. and the problem will be solved...
(it's simple to adjust : when your AI start doing shit she shouldn't do, you have crosses the difficulty line you shouldn't have pass.)
and by the way you should rework all your stupid AI.
increasing all their stats to increase the difficulty is a freaking bad move.
cap the damage, cap the fire rate, cap their health, cap their reaction time, just give pushback effect to elites, and generally, apply the same resriction to AI and players (so shot gunners can't OS you with pushback effect from 100m away).
number of foes is enough to increase the difficulty further more. moerover it's always fun to succeed a missions if you have killed thousand of foes.
on the contrary, succees in a mission is not fun when you died countless times against absurds foes who killed you by doing things they couldn't have done.
like (for example) being shotgunned to death, by and elite you just put on his knees in a punch combo, but who has such reaction time and fire rate, that he was able to shot you beetwin 2 punchs the exact instant where it was put on knees, and then shot again and again so quickly that the game consider you don't even have time to dodge beetwin two shots, and so you just dies absolutly powerless (and even if you could have dodge, monsters at this lvl have auto aim anyway... so it would have been of no use...) =)
but well, that's only an example beetwin so many stupid things...
change all that, start giving fun and you'll see people will play the game far more.
you think that frustration is a motivation to go on.
in a way it can be, but too much frustration + too much anger + too much powerless feeling... = ...queue problems.