Hi Markus. With this automation the trigger is the derivative sensor. You dont want the fan to be a trigger. If you have a switch that turns the fan ON then the fan will be the trigger and that is not good. I would look at having a independent switch and make that the trigger so when you turn it ON it turns the fan ON with a time delay.
I am assuming you have a toilet in the bathroom. If so maybe consider my Toilet Exhaust Fan with time delay blueprint and once you have a independent switch (maybe have a push button would be good, on / off in one push) use that as the trigger in the Toilet Exhaust Fan blueprint.
Hope this helps.