Here is 2-pences solution (once again not automatized so any additional Contact will create a potential new Birthday, so you will need to redo that… it will depend on your socal life ! 
)
Going to my NextCloud Calendar, I exported the Birthdays Calendar created in Read-Only mode using the Contacts data. I exported locally on my desktop with a different name.
I simply opened it with a basic text editor and I modified this line in order to change the title of the calendar (most easy to find it later) :
“X-WR-CALNAME:”
And after that with a simple “Find and Replace” I changed every entry called :
“-PT0M”
by
“PT10H”
So on my case every notification will be send at 10am after the start of the event and not at midnight. After that I :
Saved the new file (modified calendar opened with the text editor)
Importd it on your NextCloud Calendars
Changed the colour
Deactivated the original and Read-Only Birhtdays calendar
And that’s it !
I hope I was clear…
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