This is great. I've wanted a library for this, but a SaaS will do. :)
P.S, what you are doing with Fly.io is super cool!
We happen to be pretty good at infra, so a library / self hosted setup is usually preferable. This is partially because we prefer to monitor things ourselves, and partially because we don't want to send things like auth tokens through a third party service. We might be unique, though.
I'll reach out about Diahook, as we can also have a self-hosted offering, and I think it could still make sense for you. As you can still benefit from the rest of the offering, including client side libraries and management UI (will be fully customisable soon).