This server implements the Terralith 2.0: Overworld Evolved datapack by Starmute, it includes tons of new biomes, structures, and loot.
The overworld is fully generated out to a 24,000 block radius from spawn, so no need to worry about lagging the server while exploring.
/rtp only takes you out to a fraction of the distance to the border, so there's plenty to explore that can't simply be explored by using commands.