Yeoman Rand was actually the female lead while employed in the first half of season one, not merely a recurring character, she just wasn't in the opening titles as she was only guaranteed to appear in a minimum number (presumably 8 out of the first 12) of episodes. The status of the other supporting cast fluctuated. Discovery is currently adopting a four tier approach which is a small key cast, recurring guest stars, recurring support characters, and guest stars. Most similar to DS9. The role of the supporting characters is similar to Ogawa and O'Brien in that they might be bumped up a level in the future. Battlestar Galactica and Babylon 5 had a more complex structure with second and third tier main cast who appeared in fewer episodes but were tied to contracts. That said, Andrea Thompson asked to leave B5 because she was only guaranteed 6 or 7 episodes per season and was denied the ability to do other work in between episodes. I prefer the multi tier approach as characters can get bumped up or down as story requires and you can bump off characters while having spares ready to step up.