DAVx⁵ is the only all-in-one app for all things CalDAV, CardDAV and WebDAV! It is a fully-featured sync solution for your contacts (CardDAV), calendars (CalDAV) and your tasks (based on VTODO). The app is easy to set up and integrates perfectly with your favorite calendar/contacts app (including default apps). It can also be used seperately, if you either have CalDAV, CardDAV or only Tasks. DAVx⁵ can also be used to access your remote WebDAV files.
I’ll plug this - I use it in conjunction with Fastmail, FairEmail, and a Google account I need to keep in sync. It just works.