The two large projects I've worked as a consultant were structured in milestones.

The first client had a structure they'd used previously that I just inherited -- I believe it was agreed amount / 4, each paid for delivering 1) full spec, 2) wireframes/ux, 3) MVP and QA, 4) Beta that addressed major issues identified from 3. After each stage I did the presentation and invoiced.

I borrowed their same structure for the next gig. It worked well in both projects.

However, it was very clear going in that both of these companies were going to pay me.