Talking with another user, I thought of something else. If your number of days per week is 6, add 5+4+3+2+1 = 16 constraints activities occupy max time slots from selection, for an exact result of what you want. Each constraint is for a pair of days, selects these slots with X, and the maximum number of hours is 9. So only one day will allow 5 hours per day.
I hope I am not mistaking.
I attach an example for slightly different conditions (5 days per week, 6 hours per day, a single day with 5 hours per day).