I'd prefer a la carte TBH. if you just need a place to host your game, having a mod workshop, item marketplace, multiplayer servers, input management, etc. doesn't matter to you, then why pay for it? People who want that can negotiate it. For a base store hosting, 10-15% is probably fair.
But to be real honest I think the pricing parity is what goes too far. You want to charge 30%? okay, I should have the ability to pass that to the customer. the game is $15 but if you go to Itch or GoG you can get it for $10. But that's against steam's TOS.
no one else in the PC space charges 30%. GOG does but at least offers (or used to) an upfront payment in exchange for taking more share until it's paid back. Yeah, I think if they take that literal financial risk on my its worth giving them a larger share.