Only send alerts from the end user perspective. In your case the end user would most likely go into the backups and list them. So I would have a job that lists the backups every day and if something is missing it alerts.
See the difference here is that you don't monitor the systemd backup job, you monitor the backup backend instead. Because systemd can be configured to retry a job, the end result is in the backend.
And in other cases I do have monitoring for individual services, but I only send alerts if the end user experiences an issue. So a web server process/systemd unit is being monitored, but the alert is on a different monitor that checks if the website returns 200, or if it contains a keyword indicating it works.
See the difference here is that you don't monitor the systemd backup job, you monitor the backup backend instead. Because systemd can be configured to retry a job, the end result is in the backend.
And in other cases I do have monitoring for individual services, but I only send alerts if the end user experiences an issue. So a web server process/systemd unit is being monitored, but the alert is on a different monitor that checks if the website returns 200, or if it contains a keyword indicating it works.