You can actually migrate your DB to an external host on an existing installation. I would recommend to be on Foreman 2.1 (Katello 3.16) so you can also externalize Pulp 3. The benefit is that you don’t need to run PostgreSQL at all on the application server.
The procedure is roughly to prepare the external DB server, stop all applications, dump the current database, restore it on the external DB server and finally run the installer with the right arguments to point to the external DB server. Once you’re happy, you can remove the old DB installation.
Note that Katello starting some version (3.16?) started to use the PostgreSQL EVR extension. It’s written by Katello so it may not be available on a PaaS PostgreSQL instance.