Calendar preferred time slots feature

Hi all,
first: thanks for the great product.
I am seeking for a solution for the following use case:
given user A providing some services to other users or simply a busy manager whose timetable is always full. Given other users who book a time slot at user A, but also can book ‘other possible time slots’ in the calendar, even a time range. The other users can see that user A is booked at that time or not but they can not see who has booked him/her.
If a time slot at user A becomes free - say, a user steps back or has other urgent occupation -, the other users that booked that time slot as ‘other possible time slot’ get a notification, so they can book the time slot. First comes, first served.
Let’s assume that A gives 30 minute services. The users can mark a time range as ‘other possible time slots’ and can get a notification if any time slot in that range gets free.
Also it would be good to have simple rules: say, if a service was taken this week - no change in this week is relevant anymore.
I think it would help both sides with time management - both A and the others.
Any idea or solution directing to this point?

You should check the Appointments app which, for my understanding, has exactly been created for that purpose.