Colin, I think where you are going with the room is right on. If you want a larger image, you can always try moving the viewing position a bit closer, which is the same real-world result as larger projection. Admitting I come from a higher priority toward audio, the sonic benefits of giving...
In fact, if you're OK going back a generation, which means a receiver that only has one hdmi 2.1 (8K, VRR, 4K 120) input, rather than all of them, the Denon 4700H can still be found for $1499.
It powers 9 channels, but processes 11, so you can go to 7.2.4 in the future by adding an external amp.
One additional comment. Good audio gear can last for decades, especially speakers. My L&R speakers just passed their 30th anniversary. Do you think any piece of video gear will have even 1/2 that use life? 1/3 of it?
I'm going to encourage balance, which seems to be where you have been going.
I can't count the times I've seen people start only with the image, and make it as huge as physically possible, then move the seating back to compensate (which makes ZERO sense) and/or decide to severely hamper the...
Give yourself some flexibility with subwoofer placement. Putting them in the front corners will probably be better than how they’re shown in the image.
It depends on the layout of the room. Just like with 7.1 vs. 5.1, there needs to be a fair amount of space behind the viewing position to take advantage of it.