Most of these requirements lead us to incident response platforms like PagerDuty. Opsgenie, Grafana OnCall, etc... but most of them are either not on premise/self hosted (which is a deal breaker for us) or the features are really lacking in some areas. Grafana OnCall seemed like a good option, especially because it integrated with Grafana, but there's no severity and no integration with google chat.
Would be nice to know what other people are doing where a fully fledged IRM platform isn't necessary but there is still a need to have a central view of the alerts.
We use that in conjunction with our own product (obviously bias), Rootly.com for incident management on Slack to collaborate on any alerts / incidents.