One of the Astaro folks commented that the way to do this is to download an unencrypted backup and put it on a usb stick (I think it has to be FAT). Finally, do a factory reset, then reboot the Astaro with the USB stick in place. Voila, a clean install with no log files.
You could almost get to your desired result by changing the settings for the first threshhold to 5%, 'Delete oldest log files'.