I realized a while ago that "engagement" is very much the wrong metric to optimize. The reason people optimize for "time spent" or "number of interactions" is because it is easy to measure.

I think ultimately what you need to optimize for is much harder to measure. I think for most senarios it is "value a user can recieve for a given amount of time". When measuring this, time spent in the application is acutally detrimental unless you are increasing the value a user is getting from the application by a substantial amount.