Output based pricing system for dynamic scope

My agency is delivering UX/UI design services to a client. For every project, we agree on a fixed fee with two revisions rounds, and it works great for both parties.

My goal is to avoid hourly billing for UX/UI design as much as possible. My past experiences have been rough sometimes; the hour reports served as an invitation to micromanagement. Clients start asking why this task took so long while the seemingly more difficult one took shorter etc. Now, the client needs us to iterate on some project ideas without a clearly defined scope.

I considered working on a fixed fee per mock, but that's problematic too. For instance: Should a mock that displays the same piece of UI but with expanded dropdown count as a new mock? Not sure.

Could you suggest some ideas on how to work with clients with a dynamic scope but with output-based billing, instead of input-based one like hourly billing?