I’m not totally convinced (yet) that AMP isn’t a “ranking” factor for Discover so I recommend having AMP (better safe than sorry).
The content that does well on Discover is of course totally different than what does well in Search — in Search users are looking for something specific so your content needs to promise them that in a straightforward way. Discover is all browse-based so the content that does well there might withhold information from the headline or just generally seek to inspire curiosity. There does seem to be a snowball effect: if a story gets good engagement on Discover, Google will push it to a lot of other Discover users, typically over just a day or two. You need a lot of shots on goal to get a hit.
I could believe that meta tag helps. Also make sure you have a big square (1:1) image in your structured data — if Google uses that for the story’s thumbnail in discover it will appreciate huge.
Good luck!