A space per subject
Everything starts with a space, which is one subject: a certification, a course module, a language, a talk. Keeping subjects separate matters more than it sounds, because finance, law and operations reuse the same words for different things, and a shared pool would schedule one subject's term against another's definition.
A space can be shared with other people as owner, editor or viewer through a single-use invite link, so a study group or a team maintains one item bank rather than several overlapping ones.