Well, hopefully you're not just using an incrementing user id field. Twitter's Snowflake format is much closer to "definitely unique". So then you have a very simple set, which you could very simply merge with other sets. So that might help you get started with merging those pieces of data.

But I agree, there are certain kinds of data that you can't so efficiently merge. Your best bet is to try to adapt it into some sort of known, proven CRDT. (Dunno how you do that with this database, haven't really read up on it.)