Best way to structure team

I recently started working in a state government project that is being built from the ground up. I was brought in by the communication team to champion user experience in the technology platform the project is building but have been struggling to truly find my place.

The project has people responsible for different aspects of user experience: • There is a team responsible for designing business processes, which also does user research and persona development.

• The communications team is out in the field talking to real users.

• There is someone conducting user testing and usability.

• I am responsible (for now) for copy and content.

• And many many developers, mostly contractors (if that is relevant)

The problem is that no one thinks of all this as one process and the gears are not really working together. Leadership is mostly developer-focused, and it is the developer-team leads who make many UX decisions during sprint.

They send mostly finished screens to me for “grammar review”. The result is a lot of re-work, or problems being surfaced after it is too late to make changes or tasks being delayed because solutioning was not complete.

We are now wrapping up the first phase of the project and moving into another. There is a window of opportunity to create better processes and leadership is interested in that. I want to suggest to leadership that we need more focus on upfront designs and that we should move wireframing and copy before sprint, when there is enough time for feedback and iterations (Duh, I know).
How do I sell this to stakeholders without alienating other teams, or making them feel I am crossing swimming lanes? What else should I be thinking about?