I have a setup very similar to yours. I’m the guitar player in two bands and just bought a CQ18T. I’ve used it for 3 or 4 gigs already and couldn’t be happier. My PA in one band is all QSC but in the other band it’s Yamaha DXS subs and DXR mains. In my larger band I have lead guitar, rhythm guitar, drums (drummer has his own snake and feeds the board Kick, Snare, Toms, Floor Tom, Snare), 4 vocals, keyboard. 4 monitor wedges, 1 monitor IEM, one aux-fed center fill.
As for sound quality, the channels and preamps on the CQ18T are just as good as any other A&H. If you’re wanting to improve your band’s sound, become more familiar with the various sound shaping features available to you. The more expensive mixers will give you more features like busses, matrixes, more routing options, more interface options, and a bunch of other stuff, but not better sound quality. Sound quality will be 100% dependent on the signals you put into the mixer and your own skills in getting the best sound out of those inputs.
As for why someone would need mixers with more features, well…the CQ18T is great for small to mid-sized bands playing small to mid-sized venues that don’t have a lot of complex stuff going on. But now try to imagine mixing The Eagles in Madison Square Garden with a CQ18T. 16 channels? The drums alone will probably use more than that. 6 aux outs? lol.. not even close. And a whole bunch more stuff that will require gear with a lot more features.
Now imagine gigs that fall inbetween a bar band and an Eagles stadium tour. Maybe a theater that puts on plays. Small outdoor music festival, etc.. They will have needs that fall between the bar band and the Eagles. That’s why there are so many mixers and other gear on the market.
TL;DR - if the CQ18T meets your needs as far as features, inputs, outputs, etc, and all you’re looking to do is improve your sound, focus on what each instrument is feeding your mixer and improve your mixing skills. I’m just a guitar player who has been running sound only for my own bands going on 30+ years now and compared to “real” sound engineers, I’m a complete noob, but I’m always learning and trying to improve things, much to the dismay of my bandmates who have to put up with my constant “hey, next gig let’s try this setup, or that setup…or …I need an extra 15 minutes for soundcheck to try something…”
What I do know is that for my needs the CQ18T is pretty much perfect without being so complicated that I have option overload or lose my way around the various screens. And anything that doesn’t sound good in my mix is 100% my fault.