@matteo you need to go all custom, unfortunately 
Wix apps are built for simple needs, and doesn’t allow you - most of the time - to expand the functionalities and the capabilities of your booking system the way you want it, they cannot build an app that serve all the possible needs.
If you’re willing to go custom, we’ll be glad to help you here on the forum with your journey, or you can hire a developer on the Marketplace .