I think I’ve settled on the latter. Disagreement is maybe best communicated by the absence of an upvote? And downvotes work best when they signal something that is just off base, and while not reportable, is not appreciated at a broad cultural level.
It doesn’t have to be a button. Just let users see how many other users saw their comment. If a user scrolls into the comments section and stops on my comment for a second or two, that counts as a read.
Alternatively, tally up the total number of user-seconds spent viewing my comment. Or maybe an average. Just something that lets me know I’m not a ghost!