For those seeking out a federated and open alternative to Tumblr, Wafrn is looking extremely promising. It can speak both ActivityPub and AT Protocol, offers a ton of interesting features and customization, and focuses on making an incredibly fun experience.
Oh, I misunderstood, I didn’t realize this related to Tumblr specifically…
Still, it might make some of the dev work easier, if they do end up migrating the backend over to WordPress.