Are there any data cleanup strategies implemented?

[OttoKerner]

@BurnInHell

The first Problem is, that I am missing a oppurtunity to define the place of teh data directory,

See [HowTo] An extended guide on how to move the Plex data folder on Windows

@BurnInHell

The second problem is, that it is again at roundabout 20GB

That is far from unusual with Plex. https://support.plex.tv/articles/202529153-why-is-my-plex-media-server-directory-so-large/

@BurnInHell

Both dbs com.plexapp.plugins.library.db and com.plexapp.plugins.library.db-wal are around 3,2GB.

Are you sure that these are GB? That is unusual.
Which server software version are you running?

@BurnInHell

I know about the path I can set for reencodes and database backups, which I used, but I am seeing, only the blob database is backuped into this folder. There are no backups from the other databases, not in the external nor in the original folder.

Uh oh, this hints at the database file being corrupted.

1. activate debug logging (not ‘verbose’!)
2. quit Plex Server
3. wait 1 minute
4. start Plex Server
5. wait 5 minutes
6. fetch log files and attach them here

Or inspect them yourself. Take a look at the Plex Media Server.log file and seek for messages about database corrupt or malformed.
If you find these, you may have to repair your database.
https://support.plex.tv/articles/201100678-repair-a-corrupt-database/