Actually the way Wikipedia works is slightly different: they don't redirect to a lead database, they instead call this MySQL function to wait on the replica for it to catch up:

    SELECT WAIT_FOR_EXECUTED_GTID_SET($gtidArg, $timeout)

https://github.com/wikimedia/mediawiki/blob/434c333d9b2be817...

I wonder if there's a PostgreSQL equivalent of this?