OpenTasks app
OpenTasks is Writing tool that is open source under the Apache-2.0 license. A task manager app, allowing you to categorise your todo list by urgency, state,timeframe etc.
Tasks can be synchronised with a CalDAV server using, e.g.,
https://www.openapk.net/davx/at.bitfire.davdroid/ and https://www.openapk.net/etesync-secure-data-sync/com.etesync.syncadapter/