Hi Tjark Radewaldt,
You should try to follow link https://docs.mendix.com/howto/data-models/denormalize-data-to-improve-performance
A piece of a possible solution:
You could count the number of bookings for a room using an aggregate function in a datagrid. For this, you do have to change the association to a many-to-many, refering to each other.
“today” and “tomorrow” are relative to current datetime, this is tricky, also given timezone issues. You may want to consider if that is essential enough to be worth the trouble.