I want specific settings for a modpack. currently I can do that with some workarounds but I'd prefer to do so using a global config.

Unfortunately the way I have the configs organized is how MinecraftForge has intended them to be used going forward. Have you looked into using the defaultconfig folder? It is a folder Forge created with modpacks in mind, as it allows you to create custom settings for your modpack to apply to all new worlds. I may be able to help out with this if you'd like.

Yea I can use that, a bit weird that they would go world-side, why would that be? What are the advantages?

Funny enough, this comment on this reddit thread is the best documentation I could find about how the new config system works and why it is the way that it is.

Sounds to me that they wanted to have as much separation as possible between different worlds and different servers, allowing each one to have their own configurations.