Hi Dmacfari , I sort of think you have answered your own question in that it’s the Iso you are comfortable with.
Typically I did not like much over 400 on my Canon but I feel the D500 is better coupled with that I have Topaz DeNoise giving more tolerance. So for me my limit is 3200 for normal daylight shooting.
I only shoot Wildlife but would consider going higher for something like bears at night.
I would experiment and see what you are prepared to put up with.
I think Steve mentions setting s on his Auto Iso vid on you tube
Cheerz Robin.