Ah, did not see it. I think your conclusion there "None of these measurements met my goals of being easy to measure and capturing the entire impact of mental illness." is correct and it's probably worth thinking a bit about the metrics. That said, even improvement on an imperfect metric could do a lot of good.
I like this report and it's a good start. In some ways i wonder if even this report underestimates the real costs. Mass shootings, divorce, funding of crime, poor financial choices are things that come to mind as negative flow-through effects of poor mental health.
With that in mind, I find that it is difficult to even talk about mental health interventions without a good metric. Reducing number of suicides is great, but it's not going to capture large groups of people going through "slightly depressed" to "not depressed," unless the intervention is correlated with reducing suicides as well. Things like improving self-reported happiness are probably good, but it's not clear that's captures what we mean by mental health.
I feel that the next step in improving the situation is more along the lines of figuring out metrics that we can measure that specifically target mental health.
Similarly for interventions, there is probably a large number that are hard to evaluate at this point. Interventions are generally in two categories: biological and cultural with biological being the easy one. Things like diet, promoting better sleep / less screen time. Cultural would be more tracking how communities evolve and help / hurt mental health and what can be done to promote them.
I am wondering if there has been any research into reducing suicide rates. These may include: