I agree with Elsa, at its price, the 200-500 f5.6E Nikkor on a D500 is arguably the best solution for wildlife. Some of us agree Nikon put out this zoom to meet competition from 3rd party super zooms. The PF Nikkor primes are both excellent, the 500 f5.6 PF especially. In a word, Revolutionary – I kid ye not! This is especially for what I like to call peripatetic photography.
In the heavier gear arena, I cannot comment on the Tamron but the Sigma superzooms get +ve reviews and the testimony is there to see in published images… I agree the 70-200 + Teleconverters are not advisable for reach (only 400) as well as IQ. 500mm is the minimum for many birds and one too often needs 600 or more.
If my trajectory is of any use, my start in wildlife photography was in 1984 with a 400 f5.6AI EDIF. A huge outlay back then, but it worked very well with film, until I tried it on my first digital DSLR…the MF was too slow to rely on. I finally could afford to upgrade to a Used 300 f2.8G Nikkor in 2015.
This fast 300 delivered very well for me with a D7200 and D500, and subsequently on FX models including the D850. My main criteria was (1) budget and (2) IQ and (3) AF performance with Teleconverters. And this TC Factor underwrote upgrading last year to a 400 f2.8 FL… Heavier yes but TC2 III = 800 f5.6.
I’m not qualified to comment on the Canon Extenders but Sigma’s TCs get +ve reviews. This opens up the route to a Sigma 500 f4 Sport, which gives the Nikon equivalent a hard run. So a 700 f5.6 PF that does very nicely for birds – see what Canadian Pro Brad Hill’s blogs, who reports on exhaustive testing. he’s mainly Nikkor but also 3rd party. The consensus rates the Sigma 500 Sport to be the most affordable of the excellent super exotic telephoto primes.
The ultimate telephoto solutions where cost and extra weight are not limiting seems to be the new 180-400 f4E TC14 Nikkor. I think there’s a Canon equivalent.
Based on much experience, I don’t hesitate to buy Used provided you get 6 months warranty. The UK has more options (but only if you can get there! If you’re interested, I can list the good shops) but monitor the main local outlets here in SA. We are likely to keep benefiting from trade-ins by obsessive upgraders chasing the latest. This is how I scored a almost mint D500 in late 2016, among other bargains.
Handholding is manageable with a lens of approx 3kg but not for too long. Shooting any rig weighing 2+kg needs support if you are shooting properly ie all day and following subjects for any period more than a few minutes. For on the go work, a monopod works well but get a light gimbal, as a monopod head = brutal finger crusher under a heavy lens (!) I use a Manfrotto with a modified Jobu Jnr 3 Deluxe (Gimpro foot+ drop-arm). Otherwise use a decent tripod + gimbal.