Depending on how everything plays out, I might migrate to a default mode where only the communities you’ve subscribed to is shared from your computer to allay these concerns. The current mode of share-all would remain available as well.

Then...Usenet? But a little nicer because it's text only (I don't recall the level of fetching Usenet has exactly, I might be wrong).

Your server of choice hosts Usenet, your client doesn’t fetch anything unless you tell it to.