Of course, but you end up with multiple positions that are largely the same but titled “level 1,2,3,4,5,6,7,8,9,10” etc.

For a small business, that's often pretty hard to define. How much is your first marketing hire worth? Depends on how much business they bring in and how much they can get done before you need the second one. You can't identify in advance what marketing work is to be done and how important it will be - that's what you need the marketing hire for!

I mean you've got to pay them something while you wait to find out how much business they'll end up bringing in, so you can start with that...?

Unless you're paying on contingency, in which case you can absolutely just advertise that you're paying directly on contingency and provide the formula. That seems fine to me.