Currently I shoot both a D5 and D850. I photograph wildlife, architecture, travel. My lenses are 600m/f4, 300 PF, 500 PF, 80-400 AF-S, 70-200, 24-70, 14-24, Sig15mm linear fisheye , and 105mm/2.8 micro.
I love my gear and it does what I need and want. I also consider the Nikon gear to be tools - not cherished items to be loved and kept based on sentimental reasons.
HOWEVER, I assume that when I go mirrorless, I will also invest in new lenses. Therefore, I feel free to consider Nikon, Canon and Sony in that order.
If we decide to travel in Europe I will consider either a Canon or Nikon mirrorless system.
After schlepping my 600mm/f4 tripod and gimbal around Yellowstone for a week, I realize I'm not getting any younger or stronger. IF I were to go mirrorless for wildlife right now, I'd test the Canon system, since a good friend is a Canon pro and in their Photographers of Light program. While Sony seems to be the best right now, I'd rather not go the Sony route. Thus, I'm hoping that Nikon can catch up with Sony re: focus acquisition and tracking.