The 5DSR and 90D are both excellent and different. As long as you stick either to the centre point or the 9 points, the 5DSR is very good for BIF, and the bird is often in focus if it escapes from them and still in frame. So, I prefer the full frame for locating the bird and keeping it in frame. The 90D is at least as good with those sets of points. The 90D does outresolve the 5DSR but you definitely need the best lenses and wider apertures to take full advantage of the smaller pixels. My 100-400mm II is a really good copy and very sharp edge-to-edge but the 400mm DO II has now pulled in front of it, and the TCs are showing their limitations. The smaller file size of the 90D is a real plus, and in CRAW mode a further 40% smaller - so faster processing and less storage required. At the same size output in cms, the noise is the same. But, at a single pixel level at higher iso, the 90D is slightly noisier.
We need two high resolution cameras for when we travel together. And even though I really like the 5DIV it is simply not as good as good for high resolution to get the best out of bird photography and so you need longer lenses. I would have been equally happy with a second 5DSR at current prices. What the 90D now does is to allow you to travel with just one telephoto for bird photography, in my opinion the 100-400mm II, and give you enough reach, equivalent in terms of both reach and IQ, of a 640mm lens on a 5DIV, without faffing around with extenders and be able to zoom out as well. You can do nearly as well with the 5DSR, but 1.4xTC does help more with that.