Billybob, welcome back. I too took a walkabout, mine through Panasonic and Sony lands. The one quibble I have with your post is that I don't think the Sony 200-600 is in the same league as the third party 150-600s. It may be a G lens, but I own it and the 600mm GM f/4, and aside from aperture, the 200-600 really holds its own. I've been out in the rain innumerable times, and it has been as tough as my 600 f/4. I would say that if the 100-400 II is an L lens, then the 200-600 should qualify as that level as well.
So I will be comparing it to the 100-500 with hopes that it has similar image quality and build. The reduced length and size will be very welcome. The 200-600 did suffer from one major design flaw, which was that the strap lugs were oddly placed very close to the camera mount. This made it never quite balance correctly when hung from a strap, unless you wrapped the strap under the lens foot.
The max magnification will also be very welcome. I missed that a great deal since selling my 100-400 II.