Agreed Invision can be way too feature rich. Redpen (https://redpen.io/) does an amazing job of filling that void. They boast tens of thousands of paying users, how are you planning to market based on Invision and Redpen as incumbents?
Verticalise (I know, I know, give me sh*t for creating jargon...).
Don't go after the generalist approach, focus on a niche where UX or design thinking is a growing area, own the marketing for it, and then you can move to another niche. Perhaps aim at social media agencies looking for creative approval from their clients etc, own a smaller market and grow out from there.
