My take (never having used 7200s or 8000s, or an 861 come to that) is that rear duties are relatively undemanding, certainly compared to the work the front 3 have to deal with.

As my signature will tell you, we have 3100s as rears and I'm more than happy with them. Would 5200 / 7200 / 8000s sound better? Sure thing, but given a fixed budget I'd choose to put the main money up-front every time.

Would an 861 sound better than your G68? No doubt, but specifically for surround duties, in a digital system, I'd expect any gains to be very slight. Throw in some analogue and some music listening & the gap would grow.

Finally, you refer to lack of 'free space' for the side-firing woofers of the 8000s; several people on here have 8000s (& the older 6000s) in really quite small rooms. You might be surprised at the space they can work in.

Cinema: Meridian 271, 3 x DSP640 (behind screen), 4 x DSP320 (in-ceiling), 4 x DSP3100 (surrounds), 2 x AC12 (interface). 2 x Perlisten D12s subs, Lyngdorf MP40 processor. DT 145" 2.35 AT screen. Sony VW870 projector & S-T lens, Lumagen Radiance Pro.
Lounge: Roon, NUC, Kef LS50 Wireless + 2 x REL Gibraltar-2 subs

Meridian owner since 1998