I have nextcloud as my main source for calendar, contacts, and file storage(and other stuff like notes with its apps), from there i sync its calendar and contacts with the thunderbird desktop and on android using davx5. For mail from my domain names i have desktop thunderbird syncing down from my domain host but leaving messages on the server for a certain amount of days, then in k9 i set up imap to sync those emails and i can get notifications on my phone when i get new emails.