I've not found a good solution for actual constant monitoring and I'll be following this thread, but I have a similar/related item: I use healthcheck.io (specifically a self-hosted instance) to verify all my cron jobs (backups, syncs, ...) are working correctly. Often even more involved monitoring solutions do not cover that area (and it can be quite terrible if it goes wrong), so I think it'll be a good addition to most of these.