When you take up photography, especially with an interchangeable lens camera, you should think about the "system" you are going to build to take photos: Camera, lenses, tripod, camera bag, post-processing and photo management software, flash, etc. What do you need for the type of photography you plan to do: camera resolution, maximum frames per second, camera/image stabilization, high ISO performance, and software.

If a D500 "stretches" your budget and you are looking for the most inexpensive way to get "a" lens, you haven't given this enough thought. You sound like someone who buys a nice house but can't afford furniture or maintenance. The D500 is a pro-level camera that has been optimized for wildlife and sports photography. Since you are not buying it for that purpose, you are probably wasting money that could be allocated on a "system" that will better satisfy your needs--a system that uses a lower cost camera but with one or more better lenses.

Not true! Snow looks gray if you use Auto White Balance because Auto White Balance adjusts the white balance assuming the average luminance of the scene is neutral gray--whether it actually is or not; and snow is not really neutral gray.

Furthermore, all JPEG images are adjusted in camera according to the taste of the camera manufacturer, and all RAW images are initially adjusted to some extent by post processing applications according to the taste of the application vendor. Also, because of camera and lens manufacturing limitations, it is nearly impossible that the image captured by a camera looks exactly like what the photographer saw. That is one reason for post processing. The second reason for post processing is to convert the raw data captured by the camera into art consistent with the photographer's vision of beauty (which may be totally different from your concept of beauty).

Before you buy any camera you should be able to answer several questions: What do you intend to photograph? Will you be printing large prints (greater than 8"x10")? Will you be photographing in very low light? Will you be using more than one lens? Is weight of the camera + lens an issue? Can you invest 10's or 100's of hours learning to improve your craft? Can you hold a camera steady for slow shutter speed shots? Don't allow yourself to be sold something you don't understand.

Buy the TC made by the company that makes the lens. Tamron designed their TC to work with Tamron lenses. Nikon designed their TC to work with Nikon lenses. But check to see which lenses the TC will work with. Some lenses are incompatible and can be damaged if you try to attach a TC. Some TCs protrude into the lens and some lenses have lens elements close to the mounting surface.

Great illustration of FOV and DOF!

High Sierra probably isn't your problem--but it is a good scapegoat.

Be aware that it is very difficult to hold any camera steady enough to avoid image blur at the very long focal lengths being suggested unless the camera has an outstanding image stabilization system.

You can't buy a decent tripod, including head, for $200! Tripod load capacity is only one factor that must be considered. Vibration damping is critical and is a factor of leg material and leg diameter. Vibrations are caused by wind, vehicle traffic, camera mirror and camera shutter. Maximum lens focal length is also critical because the longer the focal length, the more blur vibration will cause. The tripod within your budget will vibrate and the head will allow the lens to droop, resulting in blurry photos.

The two things that drive camera development are sensor and processor technology. Sensor technology evolves more slowly than processor technology. For example, the A7R3 uses the same sensor as the A7R2, but a much faster processor--allowing better in-camera processing for improved dynamic range at low ISO and higher frame rate. Processor speed doubles every couple of years.

Another factor to consider is how a company focuses funding for development. Nikon must release a full frame mirrorless camera soon or they will become irrelevant in about four years. That is because mirrorless cameras are rapidly approaching parity in performance with DSLRs, they offer some features that are impossible to implement in DSLRs, and they are significantly less expensive to produce. Since the D500 is still the best crop sensor DSLR, the D850 is the best full frame DSLR, and Nikon urgently needs mirrorless camera, I expect the mirrorless camera development will have priority for development funding for the foreseeable future.

In other words, don't wait for a D500 successor if you want a crop sensor DSLR.

If your iMac has at least twice the internal storage available as your images require, transfer them to the iMac. When storage on the iMac runs low, transfer your older images to an external USB-C drive that you leave plugged in to your iMac.

It makes no difference which one you choose. They all produce good results if you take care to learn them.

For best results, use a Tamron teleconverter with a Tamron lens. All teleconverters reduce image quality. Expect a 10% loss of IQ with a 1.4TC; 30% with a 2.0TC. Most people find the 10% loss barely noticeable and the 30% loss noticeable. Your autofocus system will also degrade with a TC: by 1 stop for the 1.4TC and 2 stops for the 2.0TC. You camera may autofocus with only a single AF point at f/8.

The GX85 is a fantastic camera--has won many awards. The D5500 is also a great camera. The most significant difference is the viewfinder. If you want to see exactly what your image will look like before you press the shutter release button, get the gx85. If you don't mind taking additional shots to get one that is acceptable, get the D5500. The GX85 will be easier to get good shots with if you are relatively new to digital photography.