O’Brien is a “chief,” Jonas and Cutler hold the rank of “Crewman,” and I think those are the only enlisted personnel who have lines and appear in more than one episode. Starfleet seems ridiculously officer heavy.
Is the enlisted/officer distinction different in Starfleet from traditional armed forces? And where are all the warrant officers?
The main crew of Lower Decks are officers. Ensign (arguably Cadet) is the lowest ranking officer in Starfleet. All the characters we follow in LD start as an Ensign in the first season.
As for the cadet thing, if you go to Starfleet Academy you are given the rank of Cadet and basically everyone goes to Starfleet Academy.
Everything points towards Starfleet being basically a organization of Officers with no grunts.