[compumike]

I appreciate the "How to monitor the monitor?" section. Always need a meta-monitor! :)

Hope you might give us a try at https://heiioncall.com/ and let me know if that fits. (Disclosure: our team is building and operating it as a simple monitoring, alerting, on-call rotations solution.) We have the cron job heartbeats, HTTP healthchecks, SSL certificate expiration, etc etc all in one simple package. With mobile app alerts (critical and non-critical), business hours rules, etc. And a free tier for homelabbers / solo projects / etc. :)

Edit: since you mentioned silencing things in your post, we also have very flexible "silence" buttons, which can set silence at various levels of the hierarchy, and can do so with a predefined timer. So if you know you want things to be silenced because you're fixing them, you can click one button and silence that trigger or group of triggers for 24 hours -- and it'll automatically unsilence at that time -- so you don't have to remember to manually manage silence/unsilence status!