I tend to agree with your notion of negative impact studies. We tend to favor the status quo, even if the status quo isn't sustainable in the long run.
I do have concerns about private groups bringing in third parties. In practice, these third parties would have incentives to produce whatever results please the organization that hired them. It'ss tricky to arrange conditions such that these third parties are truly neutral.
I think this is an interesting idea, though I suspect "on net the status quo is worse environmentally" is a hard question to answer w/o the aforementioned in-depth reviews (at least to an extent; obviously the current system has problems).
Expanding on this, private groups should be allowed to fund neutral third parties who act similarly to land surveyors: they can provide impact studies for projects the government has not planned. If the status quo is worse for the environment and more expensive to maintain, these “impact study libraries” could provide off-the-shelf projects that wouldn’t need extra regulatory approval. Advocacy groups, like say Extinction Rebellion, could reasonably fund “status quo” analyses for carbon-intensive infrastructure vs reasonable alternatives (mass transit, HSR, road diets, etc).