This resembles how I monitor all infrastructure I run. One of them has 150 small independent VMs, for which I had to build a custom micro service monitoring open source tool that I still use to this day: https://github.com/valeriansaliou/vigil

There’s no certificate expiration monitoring just yet, but everything else is there: poll probes (active ICMP or TCP probes), push probes (reporting HTTP API for apps), and local probes (reporting HTTP API for sub-Vigil for firewalled infrastructure parts).