Is this just a “space” in an open area? Or is it closed off?
5.1 is a very respectable way to start off. Expanding to atmos later is a good plan.
Regarding your speaker options, I would look closer at Ascend, and not at the other two. There are also other choices out there perhaps worth considering, including the ccb-8 + subs from Hsu.
Denon and Marantz have some great options for AVRs, and I would suggest stepping up to any of the models that offer the XT32 version of Audyssey. It will give you much better control of your system.
Also, cubbies and shelves aren’t necessary friendly to AV setups. Why not wipe the slate and start clean?
Anyway, there's a lot that can be done in terms of how you proceed... do you have plans for the rest of the basement already?
What is your budget?
Cheers!