Well to clarity, I wouldn’t build the whole thing at once, just put out the parts you need to get started, then add in the rest over time.
They didn’t need the entire system to be 100 % complete to get it up and going, they could phase different pieces in over time.
Waterfall-component approach. Only viable if a solid plan for ALL the components are in play. You could Agile each component for best speed, but the design itself would need to take about 9 months for a good solid peer-reviewed “Give it your best shot, shoot down my idea” brainstorm design series of sessions.
You’d also still have the issue of integration to over 90 seperate applications. If you could leverage the agencies owning the apps to deliver a certain interface, that could help your schedule.