Thanks again for everyone for your suggestions.
I decided to for the time being work with the Wimberley Sidekick (mounted on Arca Z1) and see if I get used to the side mounting concept. In the past I haven't felt it was as vibration free as using the ball head directly but then I was using a 3-series Gitzo CF tripod and now I have a stiffer 4-series, and that seems to make a big difference in a vibration free experience. What I like about the Sidekick is that one has the regular head always available for shorter focal length shots and of shots of static subjects.
With the bigger tripod, Z1, Sidekick and 500 PF I find myself quite satisfied with the experience once it has been properly set up. If I use the D850 without vertical grip, the balancing is easy enough, but if I use a body with vertical grip then I have to reverse the quick release plate to find the right balance point and I find that sometimes my right hand fingers are a bit annoyed by the narrow gap between the camera's grip and the end of the QR plate. When working with a head where the tripod collar stays below the camera, there is no such annoyance. Admittedly it is a minor point but I want the shooting experience to be comfortable. :-)
Once I had a situation where in the -20C temperatures the Arca was getting a bit frozen up and stiff and because of this I had not noticed that the head was not fully locked, and the Sidekick and mounted lens started to turn downwards but I caught it before it hit the tripod legs or anything. In some ways I find the Sidekick not quite to the same user experience as a normal gimbal head where the lens collar is below the lens. I have to be more careful when using it, for sure.
So, when I take care and use it properly the Sidekick seems up to the task of supporting the 500 PF and D850 and the ease of turning the lens up and down, and left and right is nice. And I don't have to carry a separate head for shots with other lenses. On the lighter 3-series I find that if I extend the legs, there is quite a bit of vibration but if I kneel down, even that gives a good experience. So I will probably be using the 4-series with the 500mm lens most of the time, because I like to shoot from a standing position. With knee pads shooting from a lower position is ok, but I sometimes forget to bring the pads along.
I love the experience of shooting the 500mm from a tripod; when I started using the lens, I worked with a monopod and also did some hand held shots, but I just prefer the tripod experience because one doesn't have to hold the rig while waiting for something to happen. When aiming the 500mm hand held, although it is a light weight lens, it's still frustrating to try to keep the focus point squarely on the subject. On a solid tripod and gimbal, it's just wonderful and I can wait while the tripod does much of the work.
I will probably continue to study the question of the ideal head to use, but for now I found that what I already have is quite good for the purpose, given the mentioned precautions.
Here is a recent photo from the D850 and 500 PF taken with the Sidekick and 4-series Gitzo. It was f/5.6, 1/640, ISO 1000, so a bright sunny day (the winter is starting to be quite good now, with lots of snow and some bright days). The image was cropped from the 45MP image to 3060 x 3060, so it's about 2-3x crop. As I am inexperienced with bird photography, I am very happy to get such a result in such a short time of use of the new lens. I can say that if I had purchased the 500/4 G, my back would have done a whole lot of complaining already.
