Flat subscription of $10 a month for the app store. Once a month, your phone tells the store what percentage of time you spend in apps from that store. Firefox has 40% of your time? They get $4.
Just letting developers get paid without taking a 30 percent cut, or not banning open source apps for including a Patreon link [1] would be a great start.
I don't think that any financial model can fit all usages. Except, of course, paid up front with paid upgrades (and a subscription if there is a cloud component). But that ship has sailed.