This is a topic I have recently been exploring since acquiring a new processor that has Atmos.
The couch on the back wall will certainly be problematic, you may have to consider a 7.2.2 setup. A good place to start is the official Dolby Atmos speaker setup guide:https://www.dolby.com/us/en/guide/dolby-atmos-speaker-setup/index.html
You are right, based on the info on the Dolby site, your room is a mess. But I think you can make it work.
Since Atmos is an object oriented surround codec, all the setup drawings have the Atmos speakers placed on imaginary line between the front and rear speakers, in your room that would be close to the beams. I have read that Atmos works best when the ceiling or speaker location is 8' or higher, lower than that and beaming can happen and then only the closest speaker can be heard, one solution is going with a bipole speaker.
I have been checking out the posts at the Triad Forum at AVS Forum, since the threads at AVS get huge I usually start at the last post and read backwards to get the most current info.https://www.avsforum.com/forum/89-speakers/921427-triad-owner-s-thread.html
Triad designs home theater speakers so there is a lot of knowledge in the owners thread. Dawn Gordon is a Florida based dealer and installer, her posts have real world solutions based on experience.
My ceilings are less than 8' so I am following her suggestion and using bipole Atmos speakers for my 7.1.4 setup, my couch is only 54" from the rear wall and there will a lot of stuff above and behind me, it should be amazing. I got lucky and found half the speakers I need on eBay at a great price so I only needed to buy 2 at retail. Triad only sells through a dealer network, I located a dealer that sells Triad speakers at 10% off, no tax and free shipping.https://avscience.com/
My Atmos speakers are the InWall Bronze/4 Surround Bipole, mounted in the ceiling, they are sealed and only 4" thick.https://www.triadspeakers.com/products/home-cinema/inwall-home-cinema/iw-bronze4-surround/
I have a small room, larger rooms may need the Silver or Gold depending on your budget, and yea, they are a lot more than what you were planning on spending. I found a price list here
I have read comments about Atmos speakers that bounce sound up vs. ceiling mounted and people who have done both say having the ceiling speakers are much better.
Find out more about Triad by watching the $300,000 11.4.6 dts-X: PRO Demo w/ TRINNOV Audio Altitude 32 & TRIAD Speakers!! CEDIA 2019https://www.youtube.com/watch?v=ifPzkRNbZIQ