If you want to fill the frame and do no cropping, which is rare in wildlife photography try a used D7200 to see how you get on. Another option hire one for the weekend......... I can only go of my experiences with the D7100 but I would prefer to crop the D750 over the D7100. The D7200 maybe better, although Bill doesn't think so, but Gramps does

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Looking at your photo posted, what would the D7200 bring to the photo over the D750?... It's framed nicely with detail, although some harsh highlights to breast which would be the same on both cameras. But my gut feeling, you can do more in PP with the D750 over the D7200.
Just an idea.....Although not a FX versus DX fair comparison, put your lens at 450mm on your D750 and take a photo of a fixed static subject, then take one in DX mode with your lens at 300mm to see how the results fair. Have you tried blacking out the outside area on the D750 to help with framing in crop mode?
At the end of the day you will need to decide what you're going to do with the photos and this may help your decision. Me, I print the odd ones off at home, but usually pixel peep hence getting the D810, plus I want to try more scapes, that was my excuse to Clare

, although my shoulder and elbow surgery has been unsuccessful and I'm finding camera too heavy at present, so may sell.