I would tell you to buy the D750 except you have no FX lenses.
If you are working with kit lenses & D200 + 55/200 they are just so so anyway.
D500 certainly is a better camera if you need its capability, high iso, more MP for larger prints. My d200 has made 11x14 that are equal to my D800 at same ISO.
Sure there is some very slight detail missing, but not enough to fret over. If you do sports, the AF will help. For family and scenery, not so much.
Count on replacing lenses to get the most from D500.