Fascinating conversation. We are looking for Cloudron monitoring @potemkin_ai ourselves.
Ideally you would keep it simple and self-contained enough that it doesn't need access to third-party app stores. In terms of the notifications, it would make sense to build a very simple API which would allow the external services (Rocket.Chat, Mattermost) to pick up nomon's notifications. You could include a sandbox for push notifications along with templates for the major services, and guidance on how to use it.
KIS and focused would be my choice for such an app.
The problem with many open-source people is that we want to cover every need and every possibility and leave every door open. To be everything for everybody doesn't work in the physical world, and to be honest it doesn't work well in the virtual world.