Postgres and MySQL aren't overly complicated for simple use cases.
A lot of other technologies drop complications like foreign keys which you won't miss when you start developing software but it gives guarantees you will miss sorely when you start seeing inconsistent data 6 months in.
The expensive and complicated thing about these is deployment and maintenance. But then, you could instead pick sqlite and switch to big one when needed.
Sometimes it is good to get a pickup truck ahead of time, but often a smaller less versatile car will suffice. But not quite a motorbike.
A lot of other technologies drop complications like foreign keys which you won't miss when you start developing software but it gives guarantees you will miss sorely when you start seeing inconsistent data 6 months in.