If you upgraded from the Qu-16 to the Qu-24, you would get 2 Groups that you could use as stereo outs if you switch them to mix mode (there are 4 xlr outputs on the back of the mixer for the stereo groups).
The 2 matrix outputs could kind of get used as 2 additional stereo outs, though there are some pretty big constraints; they do not have a separate button for using the faders to control what is going to them (all set up is done via the touchscreen), and the bigger one, the matrix output consists of a mix of groups 1-2 (or 4 on the Qu-32), mixes 1-10, and main LR. So if you’re able to compose a monitor mix from other monitor mixes, it could work.. but then you could probably just let people share mixes, or if they complain give them a mono mix.