[hxa7241]

The basic problem is a power imbalance, and its result is that they tell you how they pay. So any proposed solution built on the idea of you telling them how to pay seems very much in danger of failing for exactly the same reason that there is a problem in the first place.

(Perhaps the main avenue to a solution is in freelancers acting collectively somehow -- thereby becoming more powerful. If clients risk being unable to find service-providers because those clients have been blacklisted, they would more strongly feel the value in paying-up.)