Welcome to the forum @Damien_dempt
send-mail-result-output-format=Json seems to send. HTTP and XMPP have this option too (not tested).
JSON is less human-friendly though. Possibly third-party tools could help. dupReport says the following:
Supports both text and JSON status emails from Duplicati
Duplicati Monitoring gets HTTP feed. Topic talks size, but I’m not sure of status or source versus remote.
You can certainly check with the authors if you prefer a user-friendly report that also has the remote size.