WorldMagic is minecraft paper world management plugin.

FAQ

How work async world initialization?

Async initialization process

Sync initialization process

Why worldgen and dimension types can't be reloaded?

Apposite levels dimension types and worldgen requires specific registration in minecraft and becouse minecraft logic doesn't assume that it can be done while players is on a server, such actions would require plugin to send all players to reconfig mode and unload all plugin worlds so it's easier to server owners just restart their servers in such situation.