I don't think rich text editor is highly recommended.
What we really need is ：
quickly post some text and emojis
Quickly insert some photos, like jpg gif,even svg,ai
more convenient way to @ somebody
More convenient way to reply to some special text or photos
more convenient way to display photos when there's a lot
Also,if cannot make app asap, maybe you guys can make the website support pwa, pwa is good too