I have both the Tamron 150-600mm f/5-6.3 G2 and Nikon 200-500mm f/5.6 and have achieved good results with both. I have used both on my D7200, but mostly with my D500. I'd go with the Nikon glass for the following reasons: 1) while both lenses are about equal in sharpness in the center, the Nikon is sharper both 1/2 way out from the center and certainly in the corners compared to the Tamron. Sharpness was the first thing I checked when I had both lenses together; 2) The Nikon focuses faster than the Tamron especially at longer focal lengths. I believe this is because the Nikon stays at f/5.6 throughout where the Tamron goes to f/5.6 around 250mm, f/6 around 325mm, f/6.3 around 450mm, so it's letting less light in to the AF system starting around 325mm compared to the Nikon; 3) You can AF with a 1.4x teleconverter with the Nikon lens, which goes to f/8 and the center AF point on a D7200 works at f/8, but AF will be noticeably slower. I do not recommend a 1.4x with the Tamron as it takes you beyond f/8 around 325mm, so you are manual focus only; 4) Tamron specs the lens at 600mm, but it is not, it's somewhere around 540 to 550mm. Reviewers have also found this. IMHO you are better off with the 200-500mm and a 1.4x teleconverter, which gets you to 1.4 x 500 = 700mm or an effective 1.5 (DX) x 1.4 x 500 = 1050mm; 5) To make a long story short, the Tamron was soft at longer focal lengths out of the box, ... , sent it back to Tamron with my D7200, ... , it was no better when I got it back. It wasn't until after I tuned it that I started getting good results beyond 450mm.

FWIW.

I know we are talking big dollars here but there are deals to be found if your ready and willing to search. I just got the Sony 600 f4 that retails for $13,000, mint, used twice for $9,000.00. Now, I have been putting money back and I got 24 months same as cash so I am into it for about $150.00 a month for two years plus my down payment.
Don't get me wrong, their is nothing wrong with the 400 2.8, but the reach for BIF is just not their, in door sports, yes, outdoor BIF, not so much. For the weight and price, the 600 is the way to go.
The Sony a9II is NOT a big step in focusing speed from the a9. So the a9 used would make the better deal. Just sayin.

I have both the Tamron 150-600mm f/5-6.3 G2 and Nikon 200-500mm f/5.6 and have achieved good results with both. I have used both on my D7200, but mostly with my D500. I'd go with the Nikon glass for the following reasons: 1) while both lenses are about equal in sharpness in the center, the Nikon is sharper both 1/2 way out from the center and certainly in the corners compared to the Tamron. Sharpness was the first thing I checked when I had both lenses together; 2) The Nikon focuses faster than the Tamron especially at longer focal lengths. I believe this is because the Nikon stays at f/5.6 throughout where the Tamron goes to f/5.6 around 250mm, f/6 around 325mm, f/6.3 around 450mm, so it's letting less light in to the AF system starting around 325mm compared to the Nikon; 3) You can AF with a 1.4x teleconverter with the Nikon lens, which goes to f/8 and the center AF point on a D7200 works at f/8, but AF will be noticeably slower. I do not recommend a 1.4x with the Tamron as it takes you beyond f/8 around 325mm, so you are manual focus only; 4) Tamron specs the lens at 600mm, but it is not, it's somewhere around 540 to 550mm. Reviewers have also found this. IMHO you are better off with the 200-500mm and a 1.4x teleconverter, which gets you to 1.4 x 500 = 700mm or an effective 1.5 (DX) x 1.4 x 500 = 1050mm; 5) To make a long story short, the Tamron was soft at longer focal lengths out of the box, ... , sent it back to Tamron with my D7200, ... , it was no better when I got it back. It wasn't until after I tuned it that I started getting good results beyond 450mm.

FWIW.