

Given that database management systems already provide clear separation between services in the form of databases, users, and permissions, I see no need to spin up new database instances for each individual service. You say it’s easier to back up tightly coupled services and databases, but why? I find it easier to back up a single database server than multiple servers.
The real concern with shared databases is performance: some services, under certain conditions, can generate load that degrades database performance for everyone. But that’s usually a problem for large enterprises, not self-hosters.
How so? Each service uses its own database with credentials that provide access only to that database. Unless on top of a breach in your container there is some zero-day in your DBMS - which I find highly improbable - no other data will be affected.