You hit the nail on the head, I‘m mixing up a few things here… Yes, you‘re right that a body cavity would also alleviate the cellular gas and compound exchange/surface to volume issue like flatness would. And motility is an issue which is not congruent with that issue but still somehow interrelated. I guess the other problem might better be discussed in another thread (maybe in handling scale?)
Also, fair point about some cilia being so large that they‘d be visible on the starting scale of the macroscopic stage (presumably 1cm).