Great Christmas gift! Thanks!

Select the 100 best and print them. You want to be remembered for the best you can do. You don't want to be remembered as a hoarder.

Great lighting, resulting in subtle gradations of tone. I think you nailed it.

Definitely!

I have used both Capture One and Lightroom. The two programs are functionally equivalent now that Lightroom has added the new masking tool and Capture One has added HDR and panorama stitching. Capture One was easy to learn because I had learned and was using Lightroom first. Both have features that are easy to learn and features that are more difficult to learn. Capture One for Nikon will be much less expensive than Lightroom, which comes bundled with Photoshop. You might also want to look at Adobe Photoshop Elements and DxO Photolab, which are both highly regarded. Before you choose, make a list of the capabilities you want that your current photo processing software doesn't provide. That will make your selection both easier and less likely to be regretted.
https://www.pcmag.com/picks/the-best-photo-editing-software

The A7iv is for landscapes, not video. The A7siii is a video camera and a much more appropriate comparison for video. Funny that you and the reviewer weren't aware of that.

Yeah. It's a double exposure.

Love the last image!!

The only reason to keep the 5D iv is as a backup camera if you are in a workshop away from home. (i.e., you paid $$$ for a 4-day workshop in Yellowstone). Off course, as you replace your EF lenses, this will no longer be an option and it will make more sense to rent or buy a backup camera.

Yes. You missed the turn. "Fast glass" in Film days meant wide aperture only. Until relatively recently, pro cameras had no more than 20-megapixel focal planes and shutters that couldn't exceed 12 frames per second. Now, in addition to having wide apertures, newer quality lenses must be compatible with 50- to 60-megapixel focal planes and focus extremely fast. Some of Sony's new lenses use four linear focus motors to enable autofocusing to keep up with their highest frame rate cameras (30 fps). Your Film lenses aren't nearly capable of supporting the latest digital cameras.

Before you buy a new lens, photograph the same subject handheld and again on a tripod using a 2-second timer. If both images are not sharp it is likely a lens problem. If the tripod image is sharp, it isn't a lens problem and you will have the same problem with any new lens.

Fuji image looks like too much saturation and clarity have been applied. I wouldn’t expect to notice a difference in sharpness between images from these cameras unless the images were printed at least 30 inches on the long side.

Consider Olympus gear. See Thomas Stirr's web site to see what is possible with Olympus gear https://smallsensorphotography.com/category/animals/birds-in-flight. Micro 4/3 will be less expensive and lighter than full frame or AbS-C gear. Buy used if possible.

He didn't have the tools to replace and heal. He used all the tools available to him.

The Panasonic teleconverters only work with the LUMIX G LEICA DG ELMARIT PROFESSIONAL LENS, 200mm, F2.8 ASPH lens. They won't work with the 100-400mm lens. Also, you cannot use the Olympus teleconverters with Panasonic lenses.