Hello @Tanya_Salemme can you please explain this part?
Task added to project on Monday - due next Monday
Task added to project on Tuesday - due next Monday
Task added to project on Wednesday - due when exactly? Also Monday?
Because I am then thinking of working with a simple select custom field maybe that has a list of all days so those can be kind of used as trigger also and set the proper days on the rule/s
I have also found these feedback request threads that might be helpful: Rules to trigger due dates only in business days,
Set due date to a specific date,