This looks pretty good. As you pointed out, it's a crowded marketplace, but none of the incumbents seem all that invincible. If you focus on executing in your niche, I think you could do really well.

Have you thought about how you'll integrate other big social platforms: YouTube, Instagram, TikTok? I know the picture/video format requires a big expansion of features, and it might be better worth your time to refine the Medium/Twitter connectors, but it's hard for any social-promotional strategy to ignore those platforms. Even just a mechanism to format a post before copy-pasting into the network's app might be a way to approach it.