If I have a feature layer service in AGOL, what is the best way to daily, weekly or monthly edit or append new data to it?
There is an Append option for hosted feature services:
What’s new with hosted feature layers: Getting to know Append Part 1 (December 2017) | ArcGIS Blog
You could also scrip this using the ArcGIS API for Python:
arcgis.features module — arcgis 1.4.0 documentation
Retrieving data ...