Not OP, https://healthchecks.io is great for monitoring automated tasks like backup scripts. Also has the option to immediately signal failure and send an alert: https://healthchecks.io/docs/signaling_failures/
That's what I use for cron-type things. Experience has been great. I also run it as a watchdog in my alertmanager container, so I am alerted if the alerts are broken.
updown.io also has a relatively new feature called cron monitoring[0] that allows you to regularly check in to signal success. If there has been no check-in in a configured time it will alert you. For backups you could add a simple curl somewhere into your backup process to do just that.
I have found that "machine is online" is usually not what I need monitoring for, at all. I'll notice if it's down. It's all the mission-critical-but-silently-breakables that I bother to monitor.