This script will allow you to use a Google Cloud Project service account to set a default signature on all selected users using a Google App Script
Please note this script is not just a drop in place, set a couple of variables to start using it. The script requires that custom attributes (https://support.google.com/a/answer/6208725?hl=en) get created for your Google Workspace enviroment so that you can specify your setting on each user insted of creating a Google Sheet to manage all your users and settings.
Please see my blog at https://caspan.com/2021/10/google-app-script-set-gmail-signature-template-on-org-users/ for details on setting up the Google Active Script Project & Google Cloud Project and any other prerequisites.
This script require the use of the OoAuth2 library by Google. Make sure you add this library to you Google App Script library
oAuth2 Library ID - Librar1B7FSrk5Zi6L1rSxxTDgDEUsPzlukDsi4KGuTMorsTQHhGBzBkMun4iDF - Version 41
If there is a newer verison available you are welcome to use it but I have only tested with verison 41 of the Library
Here is the documentation on it the library: https://github.com/googleworkspace/apps-script-oauth2