What feature do you want to see added?

Is there a guide for this? Unfortunately, the wiki no longer exists.

Are there any alternatives?

No

Anything else?

No response

The migrated wiki can be found at https://intellectualsites.github.io/FastasyncVoxelSniper-Documentation/

If I click any link, I get Error 404

Yeah, it's just the one page as of now
You'd have to use Ctrl+F for now, until the wiki is fully migrated

Is there really only a permission, or where can I find it?

Sorry for the late reply, but based on the source code there is the one mentioned, yes.
Not sure if all brushes have separated permissions tho.

Yes they do, iirc. You can check permissions in this huge file for now: https://github.com/IntellectualSites/FastAsyncVoxelSniper/blob/9140aca06ef2ae1a6cdd72182d1a5cced16654b3/src/main/java/com/thevoxelbox/voxelsniper/BrushRegistrar.java#L86