Hey, sorry to hear that you've been having issues with the server storage limit.
I've taken a look at Regionerator, but it looks like this plugin is unlikely to reduce the size of your world because it only deletes individual chunks: https://github.com/Jikoo/Regionerator/wiki/FAQ#why-isnt-my-world-getting-smaller
Generally larger servers avoid this issue by setting an appropriate world border in their world. I would highly recommend doing this on the popular server you are talking about, as they probably should have done this from the start.
I would also suggest trying the WorldBorder plugin (not to be confused with the vanilla world border I just mentioned). The trim feature it has would be the most effective way to reduce your world size as it will remove entire regions outside of your border (which you set via the plugin, not to be confused with the vanilla world border). As always, be sure to take a backup of your server before you trim just to make sure you don't accidentally delete the wrong part of your world.