Greeeeaaat question!
Been thinking about this problem a lot and what I'd prefer myself is: 

1. You can only post chat messages to 1 group
2. But you can publish posts, articles, wikis, docs, books, git problems, etc... in multiple groups at once 
3. You can determine per group what you get notified about on your home screen
https://cdn.satellite.earth/62e3437aba0b4e51d8231807344ef056079a6a845a91594688910cde52e9b6ee.png
Since I'm giving every category an emoji-icon in the group's menu, I can reuse these as a compact way of displaying the telegram-like counters in the home screen. 
https://cdn.satellite.earth/f0b0f7d32302f4772c3128a4d97ab5897d54cf50a66499e2f669a36a6d0d879e.png

4. The home screen let's you open feeds by content category (posts, articles, ...).
https://cdn.satellite.earth/3d53d9393a6369940a33938ab9880971a078447153a56ee0c0f10860bf6e82a3.mov 

 5. MOST IMPORTANTLY: the notification count on other chats (for post f.e.) would go down for every post you've already read somewhere else. 

 Ok this one is solving the notification counter! But it means you'd have to re-order the feed when I open the community so that stuff I already read in other places falls down, and stuff I haven't read pops up, even if it's older. Right? 

 Yes exactly! Displaying the NEW stuff first within the community and letting the ALREADY SEEN / OLD stuff fall down (but not hiding it). 

 Suppose community has so many new posts (made worse by being able to post to many at once) that it's feed becomes too big to sift through. The more popular the community is, the more likely it's feed will be huge. Shall we need an algo then? 

 What's chat message vs post? Chat message has 'root'? 

 https://github.com/nostr-protocol/nips/blob/master/29.md
It's the two examples given in the NIP so far.
(though, I don't really get yet why it's not just kind 1's for post for example)
But as far as I'm concenrned it's a different use case.

Chat = "Gm frens", chronological chaotic feed, fun and giggles
Post / Thread = "Here's my thought I'd like to have an more depth conversation around) 

 Thanks, that nip is enormous, need to think it through  

 I love the icons and counters and overall the attempt to integrate different kinds of stuff under one "destination"!