My bandit system generates an ordered list of the content for each individual user. I then will track if the user came back tomorrow or churned. Yes, they may churn due to many other factors, but the signal of the content itself is strong enough.

In the past I have used the share rate to optimize, but I've realized that retention is more important.

i dont know the app but i assume he has a core action (-> which he iterates/churns out) and evaluates if people act (-> used as rating)

eg (guesses here)

new notification types "think for 9 seconds about a loved one" "what was your favorite vacation, go there"

if people act -> reward and reuse if not -> reuse less (only by random chance)

alternative to notification could be "completed medidation screen"