@Glenn_Smith says you can never have enough length and I generally agree.
I even have an old Sigma 300-800 - no IS, but on a tripod not really a deal killer and it actually is pretty sharp. But the real downside is that it is heavy, not hand holdable in any way, so as Shay mentioned, that means a large heavier tripod and a Wimberley head to use it.
The Wimberley Sidekick is fine for 400-500 mm lenses but not a 300-800. One reason my Canon 400 DO gets some love.
There have been rumors that Canon has a 600mm DO coming soon - I would love a 600mm DO simply because it would be so much lighter and smaller to handle and carry. For those who aren't familiar- "DO" is Canon's term for Diffractive Optics which replaces some of the large heavy portions of a telephoto lens with smaller, much lighter flat diffractive panels that results in a much lighter ( ~50% ) and smaller telephoto lens.
I own a 400mm f2.8 refractive telephoto and a 400mm f4.0 DO. The f2.8 is a bit sharper and a full stop faster ( that one stop really matters at sunset and helps with autofocus lock ) , and significantly more expensive, but I find myself grabbing my 400 DO often simply because it is so much smaller, lighter and more hand holdable that I'll trade off its one stop of lesser light gathering. It also focuses a bit closer too, if that matters. It does sometimes.
If you can rent the lens you are considering, I would strongly suggest it before purchasing - there are so many factors to consider, that may not be apparent to you until you have used the lens in the field a bit, that rental is a good thing to consider so that when you purchase a lens, you know exactly what you are buying.
So even though, longer is always better, it really isn't......🙀.
Weight matters, size matters, ease of handling matters, and speed of autofocus aquisition matters, - you have to find the compromises that work best for you. Especially if you anticipate travel - you can carry large heavy gear in your car or truck without too much concern, other than security. But with air travel, weight, size and security are much bigger concerns.
My Sigma 300-800 will never see an airplane. Even a 500 f4 is a major challenge to travel with.
For air travel I pretty much limit myself to a Canon 100-400v2, or the Tamron 150-600, or a 400 f4 DO. The 100-400 is very sharp, but I don't prefer its background bokeh at times, nor that of the 150-600. Despite what you might read about the 400 DO I find it a very useful lens that accepts a 1.4 TC rather nicely, but it is larger in diameter than the 150-600.
So many choices, so little time.....