postgresql – Create a Postgres 11 cluster on the production server without downtime

I have a production server running Postgres 11. The database is> 10 GB. We are using pgBouncer.
Is it possible to update to create a slave (and a group, of course) without downtime?

At this moment we have 1 unique instance of Postgres 11 and pgBouncer.
What we would like to achieve is
1. Create slave and forward some read requests there.
2. Probably change master and slave, because the current database is working on a "bad" server
3. Do some other cool stuff using a cluster

The problem here is that