Thanks!

M + Auto ISO works best when the reflectance doesn't change :)

For a situation like you describe, I just set everything manually. As long as the subject is in the same light, it works fine and you'll get far more consistent results. See page 463 (manual mode section) of the exposure book for a longer explanation.

Now, if you have variation in both reflectance and light levels, it's a different story and much more difficult to cope with - that part is covered in the Advanced Exposure Techniques of the book, page 544.

There's no ideal, just what works for you. With my D850 and D500 for wildlife / birding I use BBF so AFC, auto ISO 64 (100) - 1600, Manual, 1/1000 or faster, start with aperture open to separate subject from background, but close down for a larger DOF when needed. For a slow moving single subject, single point AF targeting an eye. For something faster or multiple targets I'll go to 9 points. The camera will usually try and focus on the thing closest to the camera. For BIF I'll go to Grp or even 3d if the bird is flying towards me. I usually use matrix metering and use compensation for dark subjects or light subjects. If you shoot raw you've got a lot of leeway to adjust exposure during PP. I usually use either a Nikon 200-500mm f/5.6 or Tamron 150-600mm f/5-6.3 G2. For sports, I sometimes use a Tamron 70-200mm f/2.8 G2 maybe with a x1.4 tele-converter on the D500 when I'm in a lower light situation.

A couple of good references are Steve Perry's "Secrets to Stunning Wildlife Photography" at BackCountryGallery.com or chapter 8 in Tony Northrup's " How to Create Stunning Digital Photography", which you can find on Amazon.