StephenJohn

Thank you Bluzman.  I've read the article and it convinced me that I need both, (cropped and full frame cameras).   I'm buying a brand new D500 for my wildlife shoots and also a used D750 to satisfy my desire for full frame (maybe some better landscapes and low-light applications).   I appreciate all of the advice I received here.
 

Okay Jerry, you've opened my eyes to the interesting possibility of the D500, not to far above my intended price range.  Thank you for that - the D500 is interesting.  The only "pro" of the D7500 is the image stabilization, but the D500 (while similar in many performance areas) has a lot more focus points and better low light sensitivity.

I hope I'm not abusing your kindness by asking for further advice.  As I'm now seriously considering the D500, should I also think about the similarly priced D750?  It's lower max ISO and less focus points, but more resolution, full frame and might be stronger at low light ISO (2956 vs the 500's 1324).  I don't understand enough about ISO to appreciate how a camera with one-quarter the max ISO capability and at the same time more than double the low-light ISO capability but since low light applications interest me, so does the possibility that the 750 might perform better there.  What do you think?  (and thanks again for all 

Thanks Jerry!  Do you think that any of the image stabilization, better processor and higher ISO of the 7500 gets "negated" by the lower (21MB) resolution?   My latest thinking is that if there isn't much of a difference (D5600 vs D7500), maybe I should go with the (cheaper) 5600 to replace the 5300 -- AND -- work toward getting A D6.  Bang around with the 5600 and take out the D6 when most appropriate?