Earlier quoted context omitted.
At least briefly consider whether or not you actually need a relational database when starting a project. I work with some rather complex data access using Postgres, JPA, Hibernate, HikariCP, and a second level cache, all within a separate microservice exposed via a REST API. To make matters worse, the responsible developer has since left the project to me. We don't even use relations, so this essentially means that…
What would Mongo give you that Postgres doesn't?
We can also easily leverage reactive streams with reactive Spring Data. Technically you could also use JDBC in a reactive manner, but it looks unlikely to work with the frameworks we use with Postgres.