Except, they didn't build the Colly in secret. They launched her in secret, possibly ahead of schedule, but that doesn't mean that they hid the cost of it. The Colossus project took 30 years and was just the spearhead of a massive public investment scheme designed to kickstart GTVA shipbuilding and R&D efforts. They couldn't keep something like that secret, even if they tried.
Also, the economic collapse was triggered by several things. One, the restructuring of the node network with increased transit times across a part of the GTVA. Two, the massive amount of civil aid that was needed to rehouse the Capella refugees. Three, economic uncertainty after the reveal that yes, the Shivans are still around and yes, they can blow up stars, and yes, the best we can do to stop them is to abandon an entire starsystem. And on top of that, we have a few months of the same uncertainty called the NTF Rebellion. Take all that together, and you get the ingredients for a tasty economic recession.
Also, remember that the Sol gate project was a direct effort to counteract said depression just like the Colossus project was.