I see finding high-quality content producers was a problem; you reference math explanations specifically.
I notice that people are usually good at providing thorough and comprehensible explanations in only their chosen domains. That being said, people are interested in subjects beyond those they have mastered.
I wonder if it is possible to approach quality content producers with the question of what content they would like to passively consume, and then try and approach networks of content producers at once. For example: find a game theory explainer who wants to read about complex analysis; a complex analysis explainer who wants to read about music theory; a music theory explainer who wants to read about game theory.
Then you can approach all three at once with the premise that if they explain the thing they are good at, they will also be able to read the thing they want to be explained well to them, on the same platform. There’s a similar trick being explored for networks of organ donations.
Also, was there any consideration given to the simple mechanism of paying people for quality explanations? I expect a reasonable core of value could be had for low cost.
I had not imagined a strict barter system or scaling of paid content; the objective in both cases is only to make up the difference between the value content producers want versus the value they expect for the first wave.
The point of diminishing returns would be hard to judge for paid content, but perhaps the two strategies could work together: survey prospective content producers for the content they want to see, and then pay for the most popular subjects to draw the rest. Once you have enough content established to draw the first wave of voluntary content producers, everything else can build off of that for no/minimal further investment.
That being said, it would probably be a good idea to keep surveying and perhaps paying for content on a case by case, say to alleviate a dry spell of contributions or if there is some particular thing which is in high demand but no one is volunteering to produce.
What about a contest with a cash award of some kind? This could drive a lot of content for a fixed upfront investment, and then you would also have the ability to select among the entries for the appropriate style and nuance, which reduces the risk of getting unsatisfactory work.