I have some experience in a room of similar dimensions using OB speakers, in fact the same thing you are using for bass. I like diffusion between the OB speakers, it always bring greater solidity in imaging, that has been my experience. I have never tried the diffusion on the ceiling but would definitely give it a shot to see what it does for you. This was my setup in a 12.5’x10.5’x8’ room:
This was a setup I tried, it failed miserably, sucked all of the life out of the room, the front wall was full of 4” of rock wool.
This was the setup I enjoyed the most, great sound in a small room. The red was/is a parabolic diffuser made of 1/8” plywood, this worked well, however I never tried any other type of diffusion in this space, I expect it would work well.
In the end you have to experiment.