That's a nice complete list Bender!
No one knows when the A6600 replacement will arrive but I'm already thinking of my next camera

I'm checking out the Canon R forum due to the hot rumor of the R8. Love the specs of the R6MKII as well. I haven't discounted the A7IV (plus OEM vertical grip) as well.
I think that the A7IV is a little too high res for action. It's got great handling for video, but 33mp non stacked is far from ideal.
The R6MKII's 40 fps ES looks very appealing considering the rolling shutter is well controlled. Another advantage is I can use my EF lenses with it with the adapter. However, it's 24MP so not enough cropping power.
Specially for tennis, I wouldn't use ES on a non stacked sensor camera, not even the R6II. Tennis really got some fast moving sticks and balls.
OTOH, the A7IV seems like a great sweet spot at 33MP. Sure it's 'only' 10 FPS but I heard the sound of the mechanical shutter at high bursts and it seems more quiet than the A6600. Also, I shoot sports using jpeg so I would imagine the buffer with a fast Sandisk UHS-II 128GB card with the 135/1.8 GM @ ISO-100-800 should be pretty much endless.
You could also do lower resolution raw (14.2mp or 8.2mp), with all the DR benefits but much smaller file size.
But for your application where silence and fast burst are more important, I think that the A9 series would be the perfect fit.
Also, I can use my e-mount lenses which are more or less more modern than if I shoot EF with the R6II. Also I don't shoot sports all the time and the A7IV is a well-rounded camera.
I'm not sure I'd recommend it if it's not used for video.
It doesn't feel like that big of an improvement for action, and resolution either, it's around 10-11% linear (7k pixels vs 6k pixels width, in example).
You're mostly paying for the stills/movie switch, the 10 bit video, the custom shutter speed increments (i.e. 1/51s) for flickering etc.
The biggest pros for my use are;
- Ergonomics +++++++++++++
- DR
- Redundant card slots
- Stills/Movie switch
- 10 bit video (more leeway with exposure)
I don't shoot birds, and for dogs I'd say the improvement isn't worth it. The a6400 already did similarly well.
Having said all the above, I'll wait for the A6600 replacement before I make my decision.
I think that if extreme DR is of no concern, aps-c is a more versatile platform for the money.