In the end I want that the SD-Card of the main computer has a long life. This can be achieved by setting the Raspi into a read-only mode. For this I installed a script by Phillip Burgess:

https://learn.adafruit.com/read-only-raspberry-pi

If I still want to change something on the Raspi, I can use one of the K30 relays to set the read/write-GPIO (defined by the script) to enable the read/write-mode.