Upgrade for d5500; D500 or D750

The problems why I want to upgrade:

  • At sunset the low light situations I need to high iso and it gives to much grain.
  • I want to be able to take more photos per second
  • I want a weather proof camera

Now the downside for me for full frame is that I take a lot of photos in zoom. For wildlife ect. Going full frame would send me back a lot with zoom reach. I want to go on safari next year and might buy a zoom with max 500mm or 600mm. But with full frame that would hardly be an upgrade The lenses I have: 50mm 1.8 18-35 1.8 18-300 NIKON 5.3-6 10-22 wide lens

What would you advise me to buy? how much improvement will d500 be with less grain on higher iso compared to d5500?