Thank you Ben. That is very useful, and is a potential option. Yes we are running ArcGIS Pro 2.5.1 alongside ArcGIS Online. So, to clarify, we could (hopefully) access the feature service from ArcGIS Online via ArcGIS Pro, run the analysis in Pro at regular intervals using scheduling / python script, and then overwrite the results in to ArcGIS Online?
I will give the maximum zoom level in the web app a go. However, I can foresee that this will still leave the source webmap or layer vulnerable for the full zoom range through ArcGIS Online as it will need to be public to feature in a public webapp.
Many thanks,
Rob