Not being able to customize tweet:card setting for a thread is disappointing. If we tweet a thread, it often ends up with a user’s avatar as the image, and more often than not the initial text in the thread does not promote the thread very effectively. The ability to create a Summary Card with Large Image would be a huge positive. I know this thread is old, but I can’t find anything else on the topic, which surprises me because promoting a site on Twitter is such a great way to attract new members and Tweets with custom cards have much higher engagement.
Right now before I tweet I have to edit an OP to insert an image I like and write an introduction, tweet it so it load the image and text I want, and then revert my edits to restore the topic to it’s original state. If I want to tweet that thread again next week, I have to do this all over again.
This is all well beyond my technical ability so I have no idea what’s involved, but, for example, on WordPress, every SEO plugin lets you set a custom twitter card and opengraph for each post. A similar option on Discourse would be really appreciated.