I think even professional Rugby players have marginal incidence of CTE compared to American Football.

I agree with you on combat sports with head strikes. But Looking at kids activities it's hard to imagine that rugby, handball, lacrosse, etc would be CTE inducing as a pre college sport activity.

Without much evidence my intuition is that this is much more specific to American Football where players are effectively using what is meant to be protection gear and using it as a weapon to strike other players.