Not suggesting it, but for the sake of knowledge you can join tables living in different databases, as long as they are on the same server (e.g. mysql, postgresql, SQL server supports it - doesn't necessarily come for free)

In PostgreSQL's case, it doesn't even need to be the same server: https://www.postgresql.org/docs/current/postgres-fdw.html

You can still do a ton of joining.

I’d start with a monolith, that’s a single app, single database, single point of ownership of the data model, and a ton of joins.

Then as services are added after the monolith they can still use the main database for ease of infra development, simpler backups and replication, etc. but those wouldn’t be able to be joined because they’re cross-service.