I shoot mostly grandkids sports with a d500 and an 18-300 3.5-5.6 Nikon lens. With good sunlight I can get some pretty sharp action pics (usually at f8 or so). However, in late afternoon light and under stadium lights using auto iso and f5.6 I get some acceptable but somewhat noisy pics. I’m considering investing in a 70-200 2.8 lens (probably the Tamron G2), but would like to know of anyone’s experience shooting action shots at f2.8 or f4. I’ve gotten pretty good at hitting my focus points using f5.6 to f8, but I wonder if I can at f2.8 or f4. Any advice would be appreciated.

Just do it. Working with a 70-200 on a D500 is like working with a 135-300 on a full frame. There's a lot of magnification! You'll need a fast shutter speed for action, at least 1/320, even with VC on (which helps ONLY with camera shake, not subject motion blur).

f/2.8 can be the difference between manageable exposure and a missed opportunity, or between reducible noise and unacceptable quality loss. At f/5.6, on the 18-300, you need an ISO two stops faster, or a shutter speed two stops slower (longer).

The other reason to use a fixed aperture, 70-200mm f/2.8 zoom is optical performance. A "3X" zoom (24-70, 70-200) is usually MUCH sharper and performs better, overall, than a super-long-throw zoom. The 70-200 range is used by most professionals, often wide open, for shallow depth of field and subject isolation.

The 18-300 lenses are really made for travel use, outdoors, in bright light, where you can stop down to f/8 or f/11. Everything about them is a compromise to save space, save weight, travel light, and not have to change lenses. You sacrifice low light performance and optical excellence for practicality and weight savings.

I shoot mostly grandkids sports with a d500 and an 18-300 3.5-5.6 Nikon lens. With good sunlight I can get some pretty sharp action pics (usually at f8 or so). However, in late afternoon light and under stadium lights using auto iso and f5.6 I get some acceptable but somewhat noisy pics. I’m considering investing in a 70-200 2.8 lens (probably the Tamron G2), but would like to know of anyone’s experience shooting action shots at f2.8 or f4. I’ve gotten pretty good at hitting my focus points using f5.6 to f8, but I wonder if I can at f2.8 or f4. Any advice would be appreciated.

If you expect your focus point to be in focus @ f2.8 you must do some focus testing/calibration of the lens to your D500 ! After successfully doing that, make sure you have appropriate shutter speed - the closer you zoom - the MORE shutter speed.

Single point AF will be your most accurate in lower light - IF you can control it properly/accurately !
I will experiment. Have you tried any FF lenses with Nikon 500? Is their a relationship between the 500’s crop sensor and the lens?

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