What a beautiful shot!Another fox image taken with OM-1+ 150-400 Pro zoomView attachment 52856
You mentioned low light conditions which is understandable. I think I would give pause to considering lenses such as the Nikon 800PF or some of the super zooms from Nikon and Sony as you won't be picking up any speed. The 500PF is F5.6 and that is without the converter you will likely need.
I would think if you want the advantage of the full frame sensor you are likely looking at the faster F4 primes vs the more compact slower primes of Nikon. I think either system delivers results but iso and shutter speed are the same no matter the sensor size. Considering that if you crop you magnify grain which can often be overcame by software. Considering this and that higher MP cameras tend to struggle a bit more in the noise department the glass becomes most important. If needing a 600F4 for example Nikon has a new sexy lens at a significant price. Sony 600F4 is cheaper, a little lighter but you can also find them in the used market.
I guess with saying all that we know you are using a zoom with built in TC but what focal length do you use the most? What is the MM for the shots of the fox? For me I would rather use a M4/3 and fill the frame than have a full size sensor that I am heavily cropping to deliver the same subject size.