I like the portfolio theory of play. The one thing I don't have a good feel for is, when is it time to let your kids exploit rather than explore? By which I mean, really start to dig into something they click with and obtain mastery?

Seems really hard to tell, and perhaps we should just be skeptical of "easy" / "dopamine hooks" things like games, and perhaps unskeptical of hard things like science, art, etc. -- if a child wants to spend lots of time on a hard thing that's probably a good sign, if they want to spend lots of time on an easy thing that may or may not be good.