I would redesign the UI - move the avatar on top of a message so I could have more screen real estate for displaying the actual message. If you need, you can leave the thin gray thread left for replies.

And increase the character limit to 5000 plus enable bold and italic fonts.

You can also offer different sans-serif fonts for messages to increase readability and include options to display math.

You can also move avatars in the main thread on top of the message along with the name, id and time. Just leave the grey thread left from the message. On that way you will increase space for reading messages.

I am telling developers what is the best from the UX point of view.

Please change the blue sky image of clouds to some cloud graphics at the entrance of the app.

Then add end-to-end encrypted DM.

Then change the type-in window at the bottom to appear only after the user clicks reply.