Another ex redditor here. The issue seems to be that a lot of people created communities but never bothered to post something. Even my little ubuntu server community has nearly 90 subscribers by now.
We should work on more visible „you‘re here, what to do next“. Something like „go to communities tab, all, subscribe to each one you like“, missing any? Make them yourselves, but dont forget to post on them since very few people will subscribe to an empty community. 10-20 posts over a month should be a good start. Generally avoid bots since they dont boost interaction at all (my personal impression).
Or a reminder for people who have made communities but no posts. That would stress them a bit I suppose but I thought I‘d bring the idea to the table.