I think Ryan is right, it would serve you better if you start your own thread, but for now let me try a point by point approach to your questions based on my limited knowledge and experience.
Most likely there won't be any Denon flagship receiver for 2020, Marantz's may offer one but may still be limited to 11 channels like the SR8012 (just my educated guess). The year is still young so you are going to have to wait and see.
If you don't want to wait, I think you can probably secure a AVR-X8500H at very decent discount now that some of the 2020 models are out, albeit those are non flag ship models. That unit should be able to power all 13 speakers but for peace of mind you can simply add the MCA 325 to power the FL/FR/C speakers. It would definitely be an upgrade to the SR8012 in terms of sound quality related specs., on paper anyway, better specs won't likely mean audible benefits because the SR8012 has distortions and other specs below the threshold of audibility already, unless you push it to beyond its limits. One caveat is, while Denon promised upgrade to 8K HDMI 2.1, you have to send it out to get done. But do you really need 8K, and do you trust any first generations so called 8K ready? I don't, based on what happened to those so called 4K ready devices back a few years ago.
Questions: 1) your SR8012 has 11 channel amps so why would you plan on adding 3 external amps? Even if you do, you can't run 13.1 either, how?
2) your future AVR can of course have 13 channel amps, but then again, why would you need 3 external amps? If you want more headroom for the front channels, you only a 3 channel amplifier such as the Anthem MCA325 or Monolith 200 WX3, so what am I missing?
I am sure large room is a factor, but it may be true that some people just believe more is better.
Here's a link to some 13 channel configurations for you to choose from:
manuals.denon.com
Again, click on the link above to see some options. In additional, Dolby's website also provides some examples:
Learn how to place your 7.1.6 overhead speakers enabled with Dolby Atmos to optimize your home theater experience. Enjoy your favorite entertainment in Dolby.
www.dolby.com
The 7.1.6 mounted overhead speaker setup guide provides speaker placement guidance to get the most out of your Dolby Atmos enabled home theater.
www.dolby.com
Dolby's actually provided the recommended positions so that should answer you questions about distances. See any example below, copies paste from the Dolby support article in the first link above.
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