Your equipment is excellent... QSC K12 speakers, can't really
beat them. Sub is nice though very little is needed in a small space. X32
console... has all the control you need.
Your problem sounds like a very simple lack of mixing ability and finess. You need to pay close attention to the volume levels you are setting for your different sources. Yes it is very typical for background music to drown out live vocals and instruments. Solution? Turn down the background music when there are live vocals and instruments!
Also turn the sub
level down A LOT in general... like to just barely be there doing anything. Most of your sound sources probably don't have much sub frequency anyway. If that thing is cranked up, your just putting a bunch of high energy low frequency rumble out into a small space where it has nowhere to go and dissipate. Turn it down. Also
roll off the low end on mic channels. A sub is only needed for dance music and certain sound effects like thunder, or to reinforce a
bass instrument or kick drum
in a medium to larger size venue!
Make sure the K12's are set to "Ext Sub" so that they cut their own low end. Do you have a cross over to make sure the right frequencies are going to the mains VS the sub? You can program a
crossover into the X32 although it's a fairly complicated and maybe beyond your
current handle on the board.
Your issue really sounds like you just need to focus on setting proper levels of your 'mix', and making slight changes (usually lowering something that is covering up something else you want to hear in that moment).
As you get more advanced you can start using compressors to assist with controlling some volume levels, and EQ to cut out problematic frequencies that are overpowering the room. But that all comes after you're able to actually set good 'balanced' volume levels between your various sound sources. Certainly adding more speakers will make your problem much worse. You can actually do everything you need with those 2 QSC K12's (less really is better).