As long as your 5.1 is of same signature (which in your case are Q-Acoustics), you covered 95-100% of your sound game set.
Case in point: Even Netflix/Prime etc carry 5.1 content. This essentially implies, most content source is recorded/broadcasted in at best 5.1 configuration. So you’re well covered. For remaining (and acutely minority) content in Atmos, you’re using old speakers from Onkyo. I’d call it wise & prudent!!
As for 5.1 vs 7.1, the additional 2 speakers are usually deployed in very large rooms with multiple seating rows. This, I believe, is to ensure say the middle row audience don’t miss surround effect which otherwise are at backend of the room. In nutshell, it means the 2 surrounds & back surroundings basically carries same sound signals (say both will make same explosion sound) unlike diff signals gets carried by Fronts/Centre (which would be carrying differently recorded sounds). This typically means, in typical Indian sized rooms, 7.1 is NOT needed. On the contrary, it’d rather act as two-sets of surrounds, thus amplifying (read manipulating) sound pattern against it was intended to be. Why not increase decibels of surrounds if one wants to hear louder bombs than it was designed to be?? If that makes sense!!
That said, I got 3 setups currently -
1. Marantz SR7011, Oppo BDP103D, Monitor Audio Silver 6 5.0, SVS SB1000, Benq HT3050, Tata Sky, FireStick 4K
2. Marantz SR6010, Pioneer BDP 180, Jamo S606 5.1, Sony 7500f, Tata Sky, FireStick 4K
3. Apple Mac Pro/Air, Genelec 6010 Pair (at office)
However, never thought of adding 10-11 speakers by merging 2-3 systems. Each play independent of the other (speaker wise) for precisely above reasons. At max, will add Atmos, but for that I think in-ceiling are most ideal choice - inexpensive + small footprint.
My recommendation is, merely adding speakers will not create immersive sonic experience, in more cases than not it might have opposite effect with too many sound waves criss-crossing & manipulating signals in tiny space. More speakers not only means more fin investment, but meticulous planning as well.
Rather a lil room treatment will reach far better dividends. There again random, zigzag wooden/cotton things would do than to invest in fancy/branded items. Cotton is cotton, unless of course it’s an apparel & one wants Giza cotton. Hope it makes sense!!