In my opinion, there's nothing wrong with the D500 / D5 for sports. Pair either of those with the 70-200 fl-e and they are incredibly difficult to beat.... I've not shot a modern 400 2.8 on those bodies.
Sony for sports - again very capable, but once you have a capable body, it's much more about the photographer than the equipment - not a popular opinion....
For context, I use both a D5 and a 1DXii for sports, there's very, very little in it... in fact, I'd go as far to say, there's nothing in it. Both incredibly capable, the Nikon 70-200 fl-e is imo the best I've ever used and the 200 f/2 from Nikon is simply epic - I've not used the canon 200 f/2 and I'm not sure it can physically focus as fast as the Nikon - barely anything focuses like that lens does. I have used a Sony A7Riii and A7Riv both good, however, both of those are not as fast to use the others.
I don't think those sites are too pricey, if you compare the used prices things on this forum sell for and to a certain extent, eBay too, those other sites seem pretty reasonable, especially with the peace of mind of a warranty etc.
In all seriousness, what are you hoping to photograph and how often? Also, how much do you anticipate on earning from these images? I have a 400 2.8 that I rarely use, it's a nice thing to have, and I'd like to add a 600 f4 at some point.... That said, I barely use it haha. If I needed the very latest and best, I'd simply rent a 400 2.8 for the occasions that I would actually need it, then always get the latest version and send it back to not absorb the huge capital cost and associated depreciation.