If you got a stage box or expander, the cheapest being AR84, you can turn another group(s) into a 5th or even 6th mix and route matrix 2 and/or 3 through the outputs of the stage box.
Or…
You could turn the mono auxs into pseudo-stereo by having aux1 be L and aux2 be R. Then use vol faders to create a stereo spread. But your player wouldn’t have smartphone control over both at the same time.
Or…
you could use main LR for the 5th stereo IEM and use 1-2 mono auxs as your main out. That would be a difficult FOH mixing task!
Having gone through this, the SQ is way better in this application, and the SQ5 has the 12 outputs you are needing. I do like the footprint of the Qu-SB, and I have kept mine (for the present) as a backup in case my SQ5 has a fatal error on a gig.