Rsynch backup --0 Differential versus Incremental

ReadyNAS backup jobs don't do versioned backups, so your question is moot with regard to the type of versioning.  You get one current backup or one backup with everything current plus the last version of anything that has been backed up and then deleted since the backup was purged, depending on options selected.  If you want all the old files for a period of time and then purge them at some interval, you can select a time interval in the rsync options to do a full backup at some period and delete all files prior to that.

A much better solution is to have two jobs.  In the main one, don't select "Remove deleted files on target" in the Advanced options.  In another that is run periodically, do select it.  That way, the main one keeps the old files but the periodic one purges files that no longer exist and transfers new and updated ones only, it doesn't erase all files in the backup and have to transfer them all.

Note that in both of these methods, you get a purge of old files without regard to their age.  You can't set up a purge for files that are greater than some age.

You can kind of get versioning by insuring a snapshot is taken before the purge.  If using the deletion and re-write option, those snapshots will grow quite large, so it's best with the other one.