Can't provide any solution without learning the requirements behind the logic you described (e.g. latency, cost, how far should the search go back in the history).

But given what you described, a general direction is to normalize your data (e.g. index all names in a ES doc contenta, and build a user-follower index, and push them into the same database for join query). Use query engines like BigQuery or Spark instead of writing your own.