I didn’t understand that a goal of the summary is to bring users to the site. So modifying the summary to include all posts in all watched topics is probably off the table. Apologies, but I am fairly new here.
My users need a daily digest of either (a) all topics for categories they are authorized to see or (b) all topics/categories they are watching/tracking. Content filtered based on likes, views, etc. is not sufficient. They need all posts. It doesn’t have to include attachments, but if not, there must be link to the topic.