Thank you Pieter, it was nice having you and your family! Thank you for the compliment about the languages it's always nice to read that!
The apartment may be too small for you, but I had a family with 3 children and another time 4 adults that found it comfortable since there is the balcony and the big terrace... and in the summer you're mostly outside.
Regarding the extra fees, they are written in the ad and I rewrite it systematically to everyone before accepting the reservation, so you knew about it before the reservation. Usually the guests stay a week or two so that it doesn't seem so expensive. And as you said, the apartment was clean, that is a lot of work, you can not estimate that until you do it by yourself.
The apartments are private, each one is owned by a different person that lives in Germany and I work for the German agency that has contracts with them, since I live here in Corsica.