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.
Lemmy doesn’t have a concept of a post that isn’t attached to a community. It’s probably possible to post to Lemmy from Wafrn by tagging a community as it is with Mastodon.
You can follow Wafrn users from Mastodon, Misskey, Pleroma, etc…