Mann, mann mann, hier sind ja wahre experten am Werk 
Vergiss mal das mit der File Retention App. Da kann man festlegen wie lange eine Datei mit einem Bestimmten Tag aufbewahrt wird.
Was du willst ist ist folgendes in deine config.php einfügen (und zwar ist es wurscht wo):
'trashbin_retention_obligation' => '7,30',
das behält die Files im Papierkorb für mindestens 7 Tag, nach 30 Tagen werden sie endgültig gelöscht. Einfach entsprechend deinen Wünschen anpassen.
im selben Verzeicnis wie die config.php liegt die config.smaple.php
dort sind alle Funktionen die du in die config.php packen kannst dokumentiert. z.B.: für den Papierkorb:
/**
* If the trash bin app is enabled (default), this setting defines the policy
* for when files and folders in the trash bin will be permanently deleted.
* The app allows for two settings, a minimum time for trash bin retention,
* and a maximum time for trash bin retention.
* Minimum time is the number of days a file will be kept, after which it
* may be deleted. Maximum time is the number of days at which it is guaranteed
* to be deleted.
* Both minimum and maximum times can be set together to explicitly define
* file and folder deletion. For migration purposes, this setting is installed
* initially set to "auto", which is equivalent to the default setting in
* Nextcloud.
*
* Available values:
*
* * ``auto``
* default setting. keeps files and folders in the trash bin for 30 days
* and automatically deletes anytime after that if space is needed (note:
* files may not be deleted if space is not needed).
* * ``D, auto``
* keeps files and folders in the trash bin for D+ days, delete anytime if
* space needed (note: files may not be deleted if space is not needed)
* * ``auto, D``
* delete all files in the trash bin that are older than D days
* automatically, delete other files anytime if space needed
* * ``D1, D2``
* keep files and folders in the trash bin for at least D1 days and
* delete when exceeds D2 days
* * ``disabled``
* trash bin auto clean disabled, files and folders will be kept forever
*/
‘trashbin_retention_obligation’ => ‘auto’,