I’m also a blind user. I was a huge Plex fan and sang its praise to any that would listen. Then acessibility started breaking. First, like you, the FireTV app stopped announcing the name of TV shows, and then the music player stopped working with Alexa. I have stopped using Plex for this reason and am wondering if there has been any improvements. I really like the thought of using Plex and would return to it in a heartbeat if the accessibility issues were addressed. However, it has been my experience that the Plex development team not only doesn’t give a crap about us blind users, but doesn’t even take the time to respond to our request for screen reader support. This is very obviousley the case since I see you submitted your post back in October and have no replies. I posted about this very issue back in 2018 or 2019 after one of their updates broke the FireTV’s access and not one of the actual team responded to me either. It is also why I canceld my Plex Pass subscription, deleted the Plex install from my NAS and started using Jellyfin, which btw is totally accessible on FireTV.
The situation has not been changed at all, even I think it is a little worse now, as unlabeled buttons are now a different control that TalkBack cannot identify as unlabeled buttons anymore. In iOS, though, things seem to be better for the client. I’d switch to other media streaming server but it seems all others (Emby and jellyfin are tested regularly) are basically the same (although Jellyfin has a most accessible Android TV App, but their mobile clients are their web interface, which is not quite confortable tbh).