Taxes are paid by the consumer. Ideally developers should price things based on how much revenue they want per transaction, and have the 30% fee shown to the user.
If that were the case, there would be nothing unethical about a 30% tax nor a 90% tax.
Therefore, it’s not the size that’s the problem, but the fact that the tax isn’t communicated to the consumer, and also that Apple wants you to charge the same price in-app as elsewhere (though I’m not sure if that part is enforced)
So what determines how large the fee has to be for it to be unethical? I don't see any principled way to decide, which makes me doubt your argument here.
If that were the case, there would be nothing unethical about a 30% tax nor a 90% tax.
Therefore, it’s not the size that’s the problem, but the fact that the tax isn’t communicated to the consumer, and also that Apple wants you to charge the same price in-app as elsewhere (though I’m not sure if that part is enforced)