You will want to test this on a copy as I haven't tried it since the latest "Update...".
Only way I know in AGOL to modify the editing template is with Map Viewer Classic.
Once loaded, choose "Edit", select the template you want to modify and choose "Manage":
Then you will see a dropdown with Properties:
You can Copy one of the exiting templates to a new one, then modify the new template properties as desired and "SAVE CHANGES" at the bottom.
Then, exit Map Viewer Classic and do NOT save changes to the map so it doesn't kill your symbols, popups, etc.
R_