The only app I miss from Android is BirthDayDroid. This app takes birthdays information from contacts and send notifications about them. Also provides convenient interface too look at the upcoming birthdays. GNU/Linux phones also have contacts and also support birthday fields. But is there a similar app to notify about birthdays?
NextCloud with CalDav should do all that. Put the birthdays in the Contacts, get a calendar to subscribe to with their birthdays.
Edit: I have no idea what front end you would need, though. The back end is as above.
NextCloud and CalDav is for synchronization, but I use only a single phone, so my contacts are up to date. But what calendar app would you recommend. I currently using Calindori, but it does not support importing events from phonebook as far as I know.