With iOS I think it is just enough to prohibit Apple from taking 30% cut from App Store installations. Similar how Microsoft cannot ask for 30% from any app installed in Windows (unless it is done from the store). In case of Apple, AppStore is basically Windows comparison - the only way to access "internet" so to say.
I think you have that the wrong way around, artificially limiting their %cut would be a kludge (who even decides what's fair?) and it wouldn't resolve other issues related to their death grip over app distribution such as arbitrary rules and censorship.
In this case there's a perfectly usable market solution to the problem.
Apple should be allowed to ask for any %cut they want, but others should be allowed to compete on equal footing and without Apple positioning themselves in the middle of every transaction, which is how they're trying to work around the EU DMA using notarization requirements and their "core technology" fee.
