A telegram-like chat approach promotes the timeline, it works when there are no deep discussions, just a lot of first-level messages, *or* the discussions are limited to a few users.
If these two situations are mixed, the discussion (sub thread) is inevitably lost between the single messages.
So, the effectiveness of the chat context depends on the interaction's style among the users. Btw, today users are anyway more used to chat interfaces, so it could be preferred for a cognitive bias.

I would like to test it, maybe with a in-thread option to switch between the two modes, and so compare the difference live. 

 Yes! Starting bubbles from second level of replies fixes this. 
 

 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.