This is a common photographer's problem. You want to maximize your (photographic) Utility subject to certain constraints.
Your Utility depends upon YOUR preferences. No one should apply their personal Utility to recommend a certain choice someone else.
Your constraints are also personal. Most of us have financial constraints, for example, but the dollar amount is likely different for each person.
Another constraint lies with legacy equipment. Although couplers are available to use lenses from one manufacturer on cameras from another manufacturer, I am leery of such devices. Department Of Unforeseen Consequences.
I had a friend who was a very successful sports photographer (Official Photographer for the IOC, that sort of success). He was a lifelong Nikon user and had a complete set of Nikon super telephoto lenses. Very large investment.
He decided he wanted to switch to Canon (this was a few years ago) and convinced Canon to switch him out for free. He handed over his Nikon gear and Canon gave him the equivalent Canon gear for free. Without such a trade, he would not have switched, even though at that time he was a millionaire.
So legacy equipment is a strong limiting factor when making such decisions.
That being said, I suspect that if I were starting over again, I would probably go with Sony. Right now, Nikon is stronger than all others, but I suspect that in the long run, Sony will win out. That is, one of my preferences would be to suffer a little now, but gain in the long run (a guess).
As one person I encountered in the field recently said to me, "Canon is lost. They don't know where to go." There is some truth in that. Canon seems to be developing on a top-down basis, rather than bottom-up. For example, their flagship DSLR, the EOS-1Ds was recently discontinued, being replaced by the EOS-1D X. Canon was charging around $7000 for the discontinued model, but its features now longer justified such a price. It seems that what Canon is doing is to move more into video features where they can charge a lot - like $15000 for a recent model. That is, they are offering features to justify high prices, not to satisfy existing or potential customers. (Top-down instead of Bottom-up.)
Canon's new super telephotos are expected to offer superb performance, but again at Super Prices. One of the basic tenets of microeconomics is that if you lower the price, the demand rises. I don't think that Canon has the right price for profit maximization. Generally if demand rises, you manufacture more and manufacturing costs per unit decline (fixed costs are recovered over a greater number of units plus gains from economies of scale).
I am new to Birding and am also retired from the workforce. I find this to be a nice hobby that gets me outdoors, gives me exercise and engages the brain in devising "hunting" strategies. There is a rising population of recent retirees and I've heard long-time Birders comment (complain) about the number of new Birders out and around with cameras.
If Canon and other companies did a smart market analysis on their customers and potential customers, they might adjust their marketing approach to accommodate this new market driven by recent retirees who are engaged in a hobby where light super telephoto lenses are VERY nice to have.
If I had no constraints and given MY set of preferences, I would choose the Nikon from your set of choices. You have to understand your own preferences (Utility) and constraints and make your own decision.