Haven given it some more thought, I think the issue is, that what I was looking for was specifically “take me to the next top level comment”. Not “collapse current thread”. In actual effect, it doesn’t make much difference. Nevertheless, it is like taking a detour to get to the destination.
Perhaps both (collapse thread; jump to next top-level comment) could be swipeable options?
Maybe a floating button, visible when viewing posts with comments, could be seen when “jump to next” hasn’t been chosen as a swipe-option? That way, the option will be clearly in place out of the box, but won’t be an eyesore for those who have set up a swipe for the action?
Very nice indeed!