Very sorry to hear about your eye issues. Good luck with treatment and recovery.I was in my local camera shop today and on a whim took a look at all the rentals (not just the Z9, like before): R3, Z9, A1.
The Z9’s finder was again initially flickery, then the flickers went away after a bit of viewing. Image is grainy compared to the two others. R3’s finder was ok, but the body was sublime. I thought the A1’s finder was very nice, best of the bunch, and the thing is tiny. I could see how the lack of weight would really help hand-holding BIF.
The EVFs of mirrorless cameras have a lot of technical decisions and settings at work. I can't speak to the specifics of you r comparison, but did want to share some facts.
The Z9 is a real time readout from the sensor that refreshes up to 120 times per second. The image is full size all the time - no reduced resolution or skipping frames. You can choose whether you want a view of the image to be created (within limits) or to view the scene without applying settings and it looks like an optical viewfinder. In low light, these factors mean your EVF can be grainy because it's using the equivalent of a very high ISO view to give you a real time image refresh., but you can turn that off if you want.
Other cameras have taken different approaches. Rather than a real time fast refresh, they decrease image resolution and skip frames. With downsizing and less data throughput to the EVF, you get a cleaner view that is acceptable, but you get less detail and less real time response. There are situations where you won't see the difference, but also situations where you will see a difference if you know what to look for.
The Z9 body is designed to be a flagship style. The larger battery and body designed as a heat sink are important features for high frame rate and 8k video on a sustained basis. The upcoming smaller version of the Z9 is expected to incorporate much of that technology, but will very likely have a reduced frame rate and 8k video limitations because of the smaller battery and more limited heat sink. That's the nature of the differences between a flagship body for professionals and a high capability body for professional use without those requirements.