Hi,
I would also suggest going for the 4 Atmos channels, no matter the ear level channel count. So between a 7.X.2 and a 5.X.4 I would suggest the 5.X.4 as well. The immersion of two Atmos channels is far more than what the 2 ear level extra surrounds do and when Atmos is only 2 channels its very "voice of god" like, not really directional, so its weird. Since you have no back wall, your surrounds can be on the walls near listening position and let the Atmos on the ceiling do the rest (since you have no back wall, and a small room, one row of seating, all the more reason not to need 7 channels at ear level).
Very best,