Hi Erwin,
You are right, the monitoring stack has recently undergone a significant change and is now implemented with Prometheus + Alert Manager. Using a widespread monitoring stack should facilitate integration and customization development.
With a Subscription (if monitoring is included), emails are already sent by the Nethesis portal. Without a Subscription, a custom Prometheus configuration is needed, but it is still experimental. Please refer to https://github.com/NethServer/ns8-metrics.
As an alternative to checking /run/backup-monitor.dat, you could also try querying the Prometheus APIs. There’s plenty of documentation available, and it would be a good idea to write a Howto.