They haven't even settled on a contractor yet? Maybe the problem here is that they're trying to write one check to fix all their problems at once instead of taking an incremental approach.
This kind of knowledge and experience doesn't come cheap, and we all know how much US city governments pay. I was at one point, very briefly, motivated to apply to a city job before I learned the pay is approximately 1/3 to 1/4 of what the private sector pays. The USDS routinely posts on "who wants to get hired" and the comments on that and other thread also mention a 66% to 75% salary reduction from baseline.
This is even before we get to legal liability and political risk shifting that can happen when there is a fully responsible contracted company involved.