This is the best way to update lists/domains in a hosted feature layer. The quest has been completed. Create a map in AGOL save it then open in Field Maps (the web browser version not the phone). The layer is in the Field map version. Follow steps as if you are creating a survey for field collection. That is, add the fields that have the lists you want to amend. When you add a field in the window to the right it has the option to manage lists depending on which field item you've selected. You can drag and drop a CSV with your amended list in ABC order or with updates. This will update immediately to the hosted feature layer. If you are wanting to create a new list, first create a list in the hosted feature layer because otherwise the default option for the item in Field Maps is a text field rather than radio. I've simply added "Other", saved the list, then refreshed back in field maps. The choice options should show "Other" now with radio. Drag and drop the CSV.
