There's certainly a market for "calm" learning, though you've already identified the main challenge: there's a smaller larger market for people who want to be educated than entertained, and the market for "calm education" is going to be an even smaller subset of that education market.
Essentially you're looking for the people who are saying "I want to be educated, but I'm not looking for the most efficient way to do it. I would prefer to move at a slower pace being driven purely by intrinsic motivation, rather than using extrinsic motivators in order to encourage me to move more quickly"
That market certainly exists, but it's a small enough niche that you'll likely have to be compete at a high price point to be viable. As point of comparison, Mentava costs $500/month, so if I were building a calm learning app (for an even smaller market), I would try to figure out a way justify an even higher price point than Mentava's.