lean ux – Best way to structure a team

I have recently started working in a state government project that is built from the ground up. The communication team has asked me to defend the user experience in the technological platform that the project is building, but I have a hard time really finding my place.

The project has people responsible for different aspects of the user experience:
• There is a business process design team that also performs user research and personal development.

• The communication team is in the field and talks with real users.

• There is a person who performs user testing and usability.

• I am responsible (for the moment) for the copy and the content.

• And many developers, mainly subcontractors (if relevant)

The problem is that no one thinks of this as a single process and that the gears do not really work together. Leadership is primarily developer-focused, and it's the developer team leaders who make a lot of UX decisions during the sprint.

They send me most finished screens for "grammar review". This results in a lot of reshuffles or problems that arise after it's too late to make changes or tasks that are delayed because the solution has not been completed.

We are in the process of completing the first phase of the project and moving to another one. There is a window of opportunity to create better processes and leadership is interested in that.
I want to suggest to the leaders that we need to focus more on the initial designs and that we should move to the wire and copy setup before the sprint, when there is enough time for the returns and the iterations (Duh, I know).
How can I sell this to stakeholders without alienating other teams or making them believe I'm going through swimming lanes? What else should I think?