I think Tumblr has a pretty unique style in terms of how it works. Even though it’s completely centralised, it already almost feels like it’s federated. So is there an actual fediverse platform that’s similar to it?
There are Mastodon-like sites that will allow you to make longer posts. There’s WriteFreely for written blogs.
I don’t think there’s a real Tumblr alternative yet, but I don’t know what a Tumblr alternative would even look like. Their customisation is very difficult to build reliably, so attracting a userbase would be a challenge for an open source project without a whole bunch of templates.
To be honest, Tumblr’s community is what makes Tumblr work. You could turn various Fediverse servers into Tumblr by adding more padding between posts and stacking replies horizontally rather than listing them after you click on them. The biggest difference, in my opinion, is how the community interacts with each other. You can copy Tumblr’s base UI but without many people responding to each other it just won’t be the same.
Tumblr is still working on joining the Fediverse so you may not need to switch if you can wait.
Edit: the general vibe on Firefish instances seems to be quite good, though it seems to be very much based on Japanese social media rather than western socials. It does offer more customisation and longer posts, though!
Second edit: damn this post is over a year old, I really need to rebuild my servers recommendation cache because I keep necroing old posts. Sorry for the notification!
Pixelfed?