Technically, it's a quadpod.

Adobe recommends 8GB of RAM.

Since you have an iMac at home, I recommend a MacBook Air (~$1,200) or a Dell XPS 13 (~ $950 - $1,250) with 8GB RAM and the 256GB SSD. I assume you know how to merge catalogs onto your desktop. Don't forget to consider the quality of the trackpad. Cheap laptops have terrible trackpads which make it much more difficult to edit photos. As for PC laptops vs Mac Desktops, my family typically has PC laptop problems within two years of purchase -- mainly screens and hinges. Our Mac laptops have never had such problems even after 6 or more years.

Your 85mm DX lens may work on smaller apertures.

Bracketing is taking more than one picture of the subject with the same lens from the same position. You can bracket for exposure (different exposure times) or white balance or focus point. Processing the multiple exposures to obtain the desired effect is a skill that needs to be learned and generally requires blending or masking in Photoshop to get the best results. If you are satisfied with mediocre results, you can shoot jpeg in HDR mode or bracket and use an HDR application.

A 73mm lens is Perfect! And you need to stand exactly 13ft 3in away from the closest edge of the vehicle.

Old lenses? Find an adapter and learn to love manual focus.

Both cameras have great lenses available for both landscapes and wildlife/sports. A complete Fuji landscape kit (~15mm to 200mm) with three pro-quality lenses will weigh about 5.7 lb, vs 5,2 lb for Olympus and 8.6 lb for Nikon. Fuji has better image quality at high ISO (6400), but most landscapes are shot from a tripod at low ISO. Although I own a Fuji system, I think I would have been just as happy with an Olympus system for landscape photography.

For birds in flight/wildlife/sports, I would favor the Olympus system because you can get a 840mm efov at f/5.6 for 4.5 lb vs f/8 for 5.3 lb with the Fuji system. The difference between f/5.6 and f/8 is significant with respect to AF speed.

The Fuji has 24Mp vs 20Mp for the Olympus. You can print 13x19 at 300dpi with the Olympus; 10% larger at 300dpi with the Fuji. Not a significant difference.

Tough choice. Good luck.

As for neck pain, I highly recommend a shoulder strap that goes across the body. Black Rapid makes good ones.

Fact: The primary light gathering device on a spy satellite is not a lens.

Before the D850, a lot of people bought the D500 in addition to the D810, because while the D810 was better for landscape photography, the higher frame rate and 1.5 crop factor made the D500 a better option for sports and wildlife.

I have both an 2015 iMac 27" desktop and a 2017 17" iMac Pro laptop. I prefer the larger screen, but I end up doing most of my work on my laptop because I can use it any place in the house or on the road. I have never compared performance. If there is a difference, I don't notice it.

Easy. Just take it in the house and keep its warm. A friend of mine did that and awoke one morning to find the walls green with baby preying mantis.

What do you need (want)? 400mm, 500mm, 600mm, 800mm? I wouldn't recommend a 2.0TC. Even with a 1.4TC you loose one stop of light making focusing slower and limiting the number of effective focal points. Unless you have a D500, D810, or D850, don't expect to autofocus at f/8 or beyond.

70-300mm + 1.4TC
Tamron 100-400mm lens (400mm at f6.3).
Tamron 100-400mm lens plus 1.4TC = efov of 140mm-560mm (560mm at f9).
Nikon 200-500mm lens (500mm at f5.6).
Nikon 200-500mm lens + 1.4TC = efov of 280-700mm (700mm at f8).
Tamron or Sigma 150-600mm (600mm at f6.3).
Tamron or Sigma 150-600mm + 1.4TC = efov of 210-840mm (840mm at f9).

On a crop sensor camera multiple focal length by 1.5:
Nikon 200-500mm lens (750mm efov at f5.6).
Nikon 200-500mm lens + TC = 420-1050mm (1050mm at f8).

I recommend the Nikon 200-500mm lens, with or without the TC.

For example?

I can do grads, or better, in Lr, Ps, or Capture One. I pay attention to dynamic range (using a histogram) and, if necessary, take multiple exposures to capture the exposures I cover that range. What am I missing?

I have always found shutter release cables to be a pain in the a**. I recommend you use a 3sec or 5sec delay shutter release setup on the camera or get an RF remote.