One thing I'd encourage is to build a culture of looking at graphs daily as a matter of practice. Alerts are only as effective as the knowledge of the writer, and human based detection is almost-always the first step to better alerting.

By looking at graphs on a daily basis, one can become much more rapid in being able to diagnose issues quickly, and also understand what the root cause is of the underlying issue (assuming you're measuring XYZ that broke). So many times I was able to look at a graph and go, "Something isn't right", long before an alert would go off. I wasn't magical, I just had spent a little bit of time every day reviewing graphs, so that I could pick out subtle changes across a handful of graphs very rapidly, which would have also been hard to pre-emptively build checks around.

One additional thought is to not over-alert. Alert fatigue is your greatest enemy, and needs to be avoided at all costs. If you're not reacting to an alert, you should question whether or not you need it.