It all sounds interesting, if still confusing. My contribution to the discussion would be this: Spend more time eliminating the garbage posts that bring Medium down to an embarrassing 'anything goes' level.

It might have seemed like a fine idea to let just anyone post on Medium, but if you truly want to up your game and present a site highlighting writers who are the best in their field you might have to drop the open invitation and be more discriminating about who you invite to write here. (I'm saying this hoping I wouldn't be one of those eliminated.)

There are also the thousands of articles about making tons of money on Medium or elsewhere. They read like the want ads in the back of the National Enquirer. They're not to be believed by anyone who knows better.

I don't have any suggestions for methods to cull your writers here on Medium but if you want quality writers you have to engage with quality writers. And they won't engage unless there's something in it for them. Writing for pennies while competing with scores of people producing mediocre writing wouldn't appeal to anyone, let alone those who work hard to hone their craft and are looking for ways to promote it.