Guys, once again I have to say THANK YOU to this awesome community! This time especially to @thex! The sensor you recommended arrived today and after 5 minutes I set it up and had it installed via auto discovery using the HomeduinoRFPir Sensor. 2 minutes later I created a rule that closes my skylights and it worked instantly.
I even improved it further: The sensor signals also when the rain stops - so it took me another 3 minutes to create a variable for “is-raining” that I set to “true” when the sensor signals “present” and which I updated with another rule if the sensor signals “absent” for more than 5 minutes. I’m using this variable now in my initial rule to close the skylights when it starts to rain - and open it again, if the rain stops.
I need to tweak the setup to see if the parameters and logic is realistic but I’m happy I found the solution I was looking for, spending only EUR 15 and 10 minutes of my precious time to set it up.