Interesting project! Can you explain the vision a bit more - I understand that every instance can have their own version of an article, but how would a user know which version of an article is most relevant to them to read (and maybe even contribute to)?
If I have several backends that more or less depend on each other anyway (for example: Lemmy + pict-rs), then I will create separate databases for them within a single postgres - reason being, if something bad happens to the database for one of them, then it affects the other one as well anyway, so there isn’t much to gain from isolating the databases.
Conversely, for completely unrelated services, I will always set up separate postgres instances, for full isolation.