I’ve thought about this quite a bit, actually. The main issue is that I don’t want these posts to overwhelm readers on either my site or on Tumblr. Even 500-word posts can be significantly longer than usual Tumblr fodder and interrupt the flow of the feed.
I know that I personally don’t like seeing these lengthy posts in my Tumblr feed — even if I want to read the post!
Having said that, I’m still thinking about it. I don’t love the “Read More” option either. I’m honestly just trying to make a better experience for followers, so your feedback is noted!