WriteFreely would be your best bet of what exists. However, the communities are small between instances, and there’s not any search algorithm or anything to my knowledge.
With what currently exists on the Fediverse, I think that having a WriteFreely and posting what you write to: your own Lemmy community; a Mastodon account; a Pixelfed account; depending on your needs and use, would be the best way of going about it within the current ecosystem as it exists.