We appreciate you like DAVx5 We’ve of course thought a lot about this option because it has been implemented due to a Managed DAVx5 customer request who were in need of this because their server was not able to set VALARMs in the web view. They preferred it to be in the way Apple handles this with iPhones (only auto-reminders for non-all-day-events) and we decided to be the same for now.
But we see the rare use case for birthday calendars - however this might be problematic if birthday calendars are read-only calendars. We have to look at thse cases also. Nevertheless we will consider adding a seperate option for this, but just for now we want the new setting to stabilize and then we can look further
Meanwhile you could work-around this by using Calendar apps that offer this option directly for birthdays (Business Calendar 2 has this option).