The R is a very capable camera (certainly not a beta test as someone suggested) and competes easily with the 5DMK4. It also works very well with EF lenses, of which, if the OP is budget-minded, there will be a lot of good deals around. It also works very well with the Sigma 150-600.
With regard to wildlife, the burst mode is 5 FPS, (the 5D MKiv is 6 FPS) and I have found that quite comfortable for my needs. I love shooting wildlife, but it's not my primary usage which is commercial product photography. Going to mirrorless has a learning curve, so it does take a little bit of playing around to find the right settings, but the R handles BIF and wildlife quite nicely. It's not an R5 or R6, but that is reflected in the price.
The R is also great in low light. I take images at around 3200+ quite comfortably, and it has a 30MP sensor, which really does make a difference. 45MP would be great in some circumstances, but I really don't want huge files that require more and more storage. You'll see that difference reflected in your images right away. It doesn't have IBIS, but all my lenses have IS (all EF lenses). IBIS would be nice, but I'm not going to pay for that premium, considering my usage. The EF lenses are sharper on the R than on the dslrs I own, some of which have had to be MF'd to calibrate.
The electronic viewfinder is brilliant (coming from a dslr, I can't tell you how many shots I've missed exposure on because I didn't notice in the heat of the moment!) You will see the exposure is off as soon as you look through the eyepiece and can compensate immediately.
The customisability of the R is great, with an easily parsed menu.
The adapter is comfortable to use and doesn't intrude on my shooting style in the least. The control wheel is very useful to me, as is the slider bar, which I use for changing AF points from spot to zone quickly.
I don't own a 90D, but I have seen really excellent images with it and the Sigma from many people, certainly you won't go wrong with that camera either. Although I do think the R may have better low light capabilities. I don't know enough about the 90D to be sure about that though. I'd also suggest you might be future-proofing yourself a little more with mirrorless.
All that said, learning to get closer to your wildlife subjects is key, no matter which camera you choose. As is learning the best settings for different situations. Local birders will usually know all the best spots, and learning patience, sneaking up and staying still, waiting for the perfect moment, is all part of the game.
Unlike many people, I think you should buy the best camera you can afford rather than sticking with something 'to learn on'. The problem I found was that I had to re-learn everything on the newer model anyway, so why bother? I found that by stepping up gradually through the models, I soon outgrew the older ones, they simply didn't do what I wanted them to do within a year or so of getting them. YMMV of course. I can't justify an r5 price-wise for my needs, but the R is a nice compromise.
Here's a sample from the R with the Sigma.
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