We're moving a decent sized forum (very specific niche, very happy membership who don't want to move to a subreddit or FB group) off vBulletin 4. 750m posts or so, couple thousand active users, and very image heavy (lots of attachments, think 'DIY projects with build logs' but for a niche hobby).

I like Discourse. I've used it for small private forums and support forums. Before I automatically just go with Discourse, is there anything else out there?

Flat discussion threads, good handling of image attachments (preferably allowing AWS back-end for the file storage), and preferably open-source.