Users don’t want an admin coming in and changing their posts. It’s more important to provide a positive user experience than to edit content to make it tweet better. There are lots of work arounds such as changing OP each time you weet, or asking for consent to permanently change a post, or adding a post in the middle of the thread and tweeting that post (twitter will pick up the text and image from the post but the link will take the user to the middle of the thread, which is not a good user experience). These are all inferior to something that allows customization of a the twitter card on a thread-by-thread basis.
Also, afaik, there is no way to enable <meta name="twitter:card" content="summary_large_image"> for a thread instead of <meta name="twitter:card" content="summary" />.