"If the overworld is so good, why isn't there an overworld two?"
Overworld Two (Forge) is a port of the fabric optimization mod "Overworld Two" that generates overworld and nether terrain much faster than vanilla minecraft at the cost of breaking parity: that is, the same seed in vanilla will generate differently with this mod installed!
Some basic results show that it reduces world gen times by up to 50% on servers!. To access Overworld Two generation, you can use the world type on the selection screen or use the level type "overworld_two" on servers. Nether generation is enabled by default but can be turned off in the config. You can visit the Discord server to discuss the mod and receive support.
Ore generation optimizations are global across all world types if the config value "fast_ores" is true.
To use Overworld Two and Biomes O'Plenty enable BOP's config option to make it's worldtype the default.
Disclaimer: This is a port of the original mod Overworld Two from fabric to forge. CorgiTaco(the porter) has been given consent to port this mod by the original authors(Gegy1000 & SuperCoder79). This Fork contains a pull request written by embeddedt, that adds Biomes O' Plenty support. All credits go to these people. I'm just uploading this mod, as the original Overworld Two (Forge) has not merged this commit for over a year and embeddedt himselft doesn't want to publish.