Hi Winny.
I see you are from Austria. Just wanted to quickly share a thought.
As far as I can tell, based on my experience, in Europe it's only AirBNB and Booking.com that matter (and Expedia, but they are not working with small apartments, they prefer hostels). I also have my apartment listed on HomeAway and TripAdvisor. TripAdvisor brought me zero customers in 6 months, HomeAway brought me two within the same period, and even those are only short, two day stays.
All calendars are synced using the default tools each platform provides, no need for a platform manager - although i do check and sync manually after each new booking, just to be sure, it only takes 30 seconds from my life, but at least i am sure there will be no double booking.
Right now i am considering delisting from HomeAway and Tripadvisor. We have on average over 80% occupancy rate (and 95% when it's high season, December - February, July-August) with AirBNB and Booking.com combined and that is more than enough. Possibly this is also one reason there aren't reservations from HomeAway/Tripadvisor since people searching through those sites can't find free dates. But who knows.
Just my two cents.