brief update: The 300mm pf was delivered this morning. Took it out and shot whatever happened to be at the bird feeder. I didn't need to look beyond the very first image to feel more than reassured. Lightweight, well balanced on the d750 body, and oh my, the iq beats anything i've managed with my other lenses.
I'm holding off anything else until there are a few more ff miles with it, but right but now the front runner is as unexpected as was going to a prime, namely a top quality used d810.
This arose from dan's post #19. Here are the advantages: Just one body to carry around; no tc to carry around; can shoot fx, fx 1.2 crop, or dx and select which on the fly through programming the appropriate buttons; faster frame rate and the chance to recompose by cropping in camera; and no emotional baggage of going "back" to a crop body, not even if it's the d500.
For the purpose of illustration:
these are of what appeared randomly, have had minimal pp, and were cropped to 1500 x 2000px before exporting to flickr.