Question for OP.

For the healthcheck's integration solution (email, signal, discord....), how did you go about doing that? I would love to hear about all that. I did find a section about Signal's integration.

Started with just the email integration, and added other integration types over time, one by one. A few were contributed as GitHub PRs.

The Signal one took by far the most effort to get going. But, for ideological reasons, I really wanted to have it :-) Unlike most other services, Signal doesn't have public HTTP API for sending messages. Instead you have to run your own local Signal client and send messages through it. Healthchecks is using signal-cli: https://github.com/AsamK/signal-cli