Postgres: Checkpoints Are Occurring Too Frequently
Your server is generating so much WAL data due to the wal_level set to hot_standby. I'm assuming you need this, so the best option to avoid the warnings is to increase your checkpoint_segments. But they are just that - warnings - it's quite common and perfectly normal to see them during bulk updates and data loads. You just happen to be updating frequently.
Changing synchronous_commit does not change what is written to the WAL, but rather the timing of when the commit returns to allow the OS to buffer those writes.
It may not apply to your schema, but you could potentially save some WAL data by using unlogged tables for your data rebuilds. Your replicas wouldn't have access to those tables, but after the rebuild you would be able to update your logged tables from their unlogged siblings.