30% is a case for antitrust given how big Apple App Store today is. 15% seems reasonable, 10% would be ideal.
Though it should be 15% for everybody, no artificial caps. Moreover, I believe once you become a platform there should be an independent nano-courthouse where you can appeal.
Today being rejected by Apple, Amazon, or Google platform is equivalent to the economical death penalty for many individuals.
It should be possible to pay $100 by individuals and appeal to an independent nano-courthouse if the original platform rejects or blocks you. If you win, the appeal fee is refunded and the platform has to cover the cost. If you lose, your $100 is gone.
Fee could be adjusted to your earnings, but basic mechanism should stay the same.