I've removed the mechanical timer switch from my original project setup to reduce one potential weak point. The downside is, that the Wemos D1 Mini is running all day every day now. Furthermore I've changed my project code, which was required after removing the timer switch from the setup. With the new code it's also possible now to check out the sensor measurements in a web browser on your comfy couch! :)
I've looked for a data sheet for the Wemos D1 Mini, but I couldn't find one. I've found a data sheet for the ESP8266 at least. Even with a current consumption of 170 mA (in the worst case scenario in regards to the WiFi connection) and an operating Voltage of 3.3 V it seems the project is still financially reasonable (1.47 Euro per year). I will check out the consumption of my project setup in the next few days.
Also I've found out about this waterproof ultrasonic sensor, which I wasn't aware of. I just ordered that one. I would definitely recommend this one for areas, which are colder and more humid (like a garage) to guarantee, that your sensor isn't decaying quite so fast. I will test the sensor I'm using right now though to get an idea about how reliable this one is in a harsher environment. I will do updates on this topic in the future.