Parmandur
Book-Friend, he/him
Technically, it is 16 Settings: 15.mini-Gazateers for Prime realms (some might be on the same planet) and one hub city in the Deep Etheral. There is quite a bit of Setting material between these Settings, and much of it has kept popping up (the Copper Dragon module in Dtagon Delves takes place in the pseudo-18th century African-American Southeast diaspora Setting, Godsbreath, created by the module author for Radiant Citadel). And beyond having a lot of Setting material...it is sharply new.You aren't the first to mention Radiant Citadel.
I don't own the book, so much of this is based on impressions. But as I understand it, that is not "a setting" in the Forgotten Realm or Eberron sense of a setting. it is a city, or nexus point, or soemthing, and within it are a bunch of very lightly sketched out other settings (Prime worlds, i think). Is that accurate?
And in either case I would ask -- what is 5e-y about Radiant Citadel? What 5E systems and assumptions does it lean into that makes it a 5E setting, other than having been produced at that time?