Of course it's legal, but the "C" stands for "Chief" which has a pretty strong implication that there are non-Chief Executives in the company.

If you're not at a position to have full-time employees whose sole job is to manager managers, it's unlikely "CEO" really fits.

CEO of a LLC is legal if you're in sole control of the company but yeh, owner/founder is fine too. I used to call myself Founder when the company was just me.