The acitivity feed is Microsoft Teams inbox. And adding the subject line to ones acitivty feed, would really help with scanning the list. Maybe add an option to group by conversation, so only a list of recently active conversation is shown and not a single entry for evey comment inside a conversation.
It does roll up conversations into one entry. If you are seeing a new line for every response then people are not replying to conversations :).
The only complain I have about it is that it should just show your "Last unread" on the activity feed entry and not the latest reply to the conversation, cause you always have to scroll back up on the conversation to read it if it was longer. If it jumped to your last unread instead you could continue the conversation from it.
Anyway, not every message has a subject attached in conversation so this really wouldn't work either. But the "Grouping" definitely is in, you may have to get your users replying properly.