There are a lot of good subreddits, but the nature of the experience is different from a Mastodon instance. And a Pleroma instance will also be different because the choice of Pleroma over Mastodon is conscious.
Mastodon doesn't yet have communities centered around hashtags (like #amwriting or #mbmbam, for example) like on Twitter, but it's getting there.
There's also the new 'Explore' feature introduced in this release. It will take maybe a week or two until enough people enable it, but it might be what you're looking for[1].
1 - https://mastodon.social/explore
P.S. This works on any instance running 2.7 where at least one user has the Directory feature enabled, just add '/explore' to the instance root URL.