Interesting option.
But probably it also promotes first level posts to gain visibility, to the detriment of contributing to an existing discussion (that is hidden behind the first level). 

 That's why I need those first to display like Posts,
so that they can be filtered for high signal in the first place.  

 Make sense. Sort sub-threads with a simple algo that takes in account replies_count, unique_users, zaps and reactions could fix that.