Remember the DX cameras allow a 50% apparent longer reach than an FX camera with the same lens.
So, putting the same lens on the D850 with its larger sensor the animal is smaller in the photo than when shooting the same animal with the D500 with the same lens. You therefore have to crop the D850 image in order to look the same as the D500. When you crop the D850 image to DX dimensions you reduce the pixels from ~46MP to ~20MP, the same as the D500. Cropping degrades the IQ so while the full frame images are cleaner with less noise than a DX image, when you crop FX down to DX size you end up with more or less the same image quality as the DX camera. In other words you lose the advantages of the better IQ of a full frame image when you crop it to DX size. So a cropped FX image in this instance is no better than one shot with DX with the same lens.
The only way to maintain the better IQ of the FX sensor is to use a 50% longer lens on the same animal, and now you're talking big bucks. Alternatively you can fit a 1,4X teleconverter to the lens but then you suffer a loss of light and autofocus in less than stellar light can be problematic.
Further, the D500 is superior in frame rate and autofocus capabilities for wildlife compared to the D850. In my opinion you already have the best wildlife camera body available in the Nikon stable. For anything else the Z7 is more than capable, so I wouldn't change anything if I were in your position.
So, putting the same lens on the D850 with its larger sensor the animal is smaller in the photo than when shooting the same animal with the D500 with the same lens. You therefore have to crop the D850 image in order to look the same as the D500. When you crop the D850 image to DX dimensions you reduce the pixels from ~46MP to ~20MP, the same as the D500. Cropping degrades the IQ so while the full frame images are cleaner with less noise than a DX image, when you crop FX down to DX size you end up with more or less the same image quality as the DX camera. In other words you lose the advantages of the better IQ of a full frame image when you crop it to DX size. So a cropped FX image in this instance is no better than one shot with DX with the same lens.
The only way to maintain the better IQ of the FX sensor is to use a 50% longer lens on the same animal, and now you're talking big bucks. Alternatively you can fit a 1,4X teleconverter to the lens but then you suffer a loss of light and autofocus in less than stellar light can be problematic.
Further, the D500 is superior in frame rate and autofocus capabilities for wildlife compared to the D850. In my opinion you already have the best wildlife camera body available in the Nikon stable. For anything else the Z7 is more than capable, so I wouldn't change anything if I were in your position.