I wish this was easier but it's not.
As you noted, the database and its contents don't reflect the version of the SSRS service directly. In addition, over the years, they often don't even increase the database compatibility level when you move between versions, including if you apply an in-place upgrade.
You need to find out what version the service is, rather than the database. You can query that by looking at the value returned when you open the report server URL like: //servername/reportserver
Note the example for mine: