As someone who has spent time in the C-Suite, I applaud your courage in publicly addressing criticisms head-on. I just hope you are not fighting a losing battle against low employee morale, frustrated writers and angry, impulsive investors. I have a friend who got some traction on Medium, but if I want to write for myself I would choose Substack in a New York minute.
You mentioned Substack, I’d like to question their model. With Medium they help with searching, although it needs an improvement, with Substack there is no such a thing, I have to subscribe to an author and read all his posts, but no human being can produce quality content 100% of time. I think they should change. Medium has better model, IMO
The way I understand it, Substack is literally a platform, more like Wordpress than Medium. It is incumbent on those who publish to do all their marketing, outreach, discovery, virality etc. using their own resources. Substack takes a very reasonable 10% on your revenue, so you don't have to be a programmer to distribute a newsletter, but getting the word out is up to you.
Best of luck!