(This post is from an email conversation with a friend. I thought the content might be useful for others.)

I certainly agree with your decision to move to full frame.  Wide angle flexibility and bigger and/or more pixels are great benefits.  I bought my D500 for wildlife (birds and whales) back before the D850 came out because my D810 wasn’t quick enough and I wanted the crop factor benefit for my telephotos.  The D500 was the first of the current product line with the dramatically better high ISO performance.

Since buying the D850, though, my D500 has sat idly on the shelf (next to my D810).  The D850 has the speed (9FPS with grip + D5 battery), great AF, great high ISO / low light performance, and same resolution at the 1.5 crop (well, 20MP vs 21MP for D500), but also gives me 45MP and more dynamic range.
A quick look at the DP Review review of the D750 shows it is good in low light, but not as good as the D850.  And that’s to be expected.  The D750 is over four years old!

I’m not suggesting the D750 wouldn’t be a good camera for you.  I personally wouldn’t buy one right now, though, as it is clearly due for an update, which will probably be an incredible camera when (if) it happens.  But if you want to do something now, instead of waiting, I would either bite the bullet and get a D850 (sorry) or go with your used D750 idea so you can keep a camera for your wildlife interests.  But like I said above, a D850 would do it all.  (Sorry.)  🙂

(And in support of your used D750 option, the used prices on them must be really great right now with the dramatic prices reductions on new ones.)