I’m curious about the argument for the stepper - are users frequently calling in with complaints that they missed setting something up? Does your business partner want to force the user to see all their choices before moving on to payment?
Long steppers aren’t necessarily annoying to users - but being forced to make more choices than necessary is. Surprising the user at the end of the flow with a larger-than-anticipated price based on their choices is also a horrific experience. But making something seamless to get through, with strong defaults and easy selections, and an ongoing price update, could be a great experience on mobile, where giving one task per screen is ideal.
I think you should test a linear flow vs. a configuration setup and see how your users do. You could give them a set of specs and see how long it takes them to complete the setup. You could also test task accuracy, if missed attributes are the concern. Run it like a science experiment and see if your business partner can support your findings.