Big thank you to everyone who's commented so far. This is a great forum, and it's been helpful to bounce thoughts and scenarios around with y'all.
Quick summary of where I'm at, and I'll continue to update the thread for anyone who finds this topic in the future that's in a similar situation. At this point I am committed to a 600mm f/4 over a 400mm f/2.8. The added versatility and extra stop of light is very tempting for the 400mm f/2.8, but for my location and what I'm shooting, I believe that 600mm is likely the better "base" for my needs. Running through all the possible scenarios out there—and I do have about five years of experience photographing in Yellowstone already, albeit with a slower 100-500mm or 200-600mm zoom—I feel like the few animals that could regularly fill my frame at 400mm, mostly slow-moving bison, elk, mule deer or whitetail, would be easy to reframe by stepping back in the instances they're too close. The occasional coyote running towards me on the roadside could pose a problem at 600mm, but would also pose a problem for the 400mm. If I miss a shot or two because of that, so be it; I'm not real keen on including roads in my images, anyway. Everything else is usually at a distance greater than 600mm, and I could always step back to include a little more of the scene if I'm looking for an environmental portrait. As stated already, it's much easier to step back and reframe than to continue getting closer beyond a certain point.
Nikon definitely seems to have the best 600mm f/4 option on the market right now, seeing that it has the built-in TC and offers that versatility to switch back and forth much more often than I'd be adding or removing an external TC. That makes a big difference with blowing snow and volatile weather in the park. That said, I still have my reservations about the size of Nikon gear and their relative lack of video priority when compared to Sony. I'm sure Nikon's video is more than enough for what I need presently, but I do like the eventual upgrade path better with Sony. So is the built-in TC and other wildlife lens options enough to overlook those two things for me? Not sure yet. Date the bodies, marry the lenses, I keep telling myself.
I'm not purchasing the lens for a few months, so I'm going to rent a few setups and also keep a close eye on what these companies announce between now and then. I know Canon is planning to tease the R5mII as well as the R1, so maybe they make a lens announcement with those for the Paris olympics. I do think both Sony and Canon will eventually be adding the TCs to their primes, but I can't base my decision on those assumptions. At this point I'm pretty content with the two options I have: sticking with Sony and buying their 600mm f/4 GM with the 1.4x TC, or switching to Nikon for their diverse wildlife lens options, adding either a Z8/Z9, and selling my present Sony setup.
One aspect of the decision down, now for the ecosystem choice.
