It's 300 characters. Have you considered writing it on leaflet.pub or something and linking it, if it's not a back and forth? There's already a + button in the post composer to split into multiple posts.

I think there's a few tweetlonger-type services that people have tried to make, but with atproto they can at least embed that extra text into the post (100kb limit), so the site only needs to stick around to view it.