Earlier quoted context omitted.
I'm really worried about the total amount of data each user will end up lugging around though. Also, append only => no deletes, correct?
Yes, no deletes. Since there is no company in the middle, the GDPR rules don't apply to this P2P network, so it isn't an issue legally (in Europe). This is something that will probably take some puzzling to get to a sustainable mechanism. The strategy I've heard talked about most often is to append metadata that marks it as 'deleted', so when things populate or 'gossip' further, it won't show up in the user interface…
Also, how much you download is related to how many people you follow, and text doesn't take up a ton of space.