The biggest difference is in weight. RZ920 is 35.9 lbs and the RZ1100 is 43 lbs. This tells me that the RZ1100 has a larger transformer (for better brute force power output). If your Mirage speakers were of lower impedance (such as 6 ohm or 4 ohm), then the larger transformer could help. The rated power output is not really different (135 watts vs. 140 watts), but I have found that the larger transformer can provide more control over the speakers with regards to bass/midbass.
The RZ920 does NOT do upscaling of video to 4K. All video is pass-through, so if you are playing 1080p, it will output 1080p. The RZ1100 does have upscaling, but it is difficult to tell whether or not it is better than what is inside your tv.
RZ920 is a slightly newer platform and has a couple extra features (newer core firmware and support for HDR features.
RZ1100 has discrete subwoofer outputs that can be processed separately (not sure if this matters to you).
Other than those things, they are pretty much the same. I would say that if you could afford it, go with the RZ1100. It is closest to your 809 in weight, and it is possible that there might be a slight reduction in sound quality with the lighter RZ920.