1) Look through posts and just and tally which "side-topics" show up the most within the last week. Rank them. The top two side-topics for each group will be the most promising ones to suggest to that group in the future. You'll basically be making a 'word cloud', like this: https://www.jasondavies.com/wordcloud/

2) To test your new hypothesis, occasionally start making your own posts on the groups with links to articles about the most promising side-topics you've identified. Either make brand new posts, or cross-post from other groups. See how popular the posts are.

3) If not, move to the next most popular side-topic and do some tests with that.

4) Repeat steps 1 - 4 (each time you do step 1, use the most recent week of posts)

If you'd like more detailed advice on how to do this, and test its effectiveness with regard to your specific groups, let me know,

best,

Lee