How can I set multiple reminders for a contact’s birthday?
iCal calendars have very limited events notifications setup on iOS, not to mention Birthdays Calendar. There are some third party solutions that can add multiple alerts. Still you would have to go through each entry and add preferred notifications. I haven’t found any that can assign in batch on import.

I have around 70 birthday entries, so this would do as a workaround for me:


Export .iCal file from Birthdays calendar on Mac,  
Add that file to Google Calendars and add preferred notifications to each birthday event 
Connect Google Account in Settings of iPhone, toggle Calendars to sync 
Set Birthdays not to display in iOS app