Basically I'm looking for a similarly permissive platform with a large audience to upload my free stuff to, and one where I can promote existing work and upcoming projects.

The problem is a lot of other platforms are extremely niche, unlike tumblr, which also had the benefit of connecting me with other creators and introducing my work to newcomers.

Gurochan and R34 sites are very niche, and most people won't have the stomach to browse them, as they are obscure and most people will find the content repulsive. Nonetheless, a lot of my stuff's going to end up here, as this is obviously a place for it.