I have no idea whether or not this would be easy or difficult to incorporate into the Duplicati backend, but there’s an open-source program called Apprise that allows notifications to be sent to basically every notification service in existence, e.g. Pushbullet, Join, Windows 10, etc. Here’s the link: GitHub - caronc/apprise: Apprise - Push Notifications that work with just about every platform!
I don’t know about others, but I would love an error-only notification popping up on my phone or wherever rather than an email at the end of a run. If it’s possible to incorporate this program or something like it, I think it would be a great addition to Duplicati.