How to fix the requirements in the release of the project?

Question

Project management

The project is being developed for quite a long time, because customers are constantly adding more and more new tasks, which leads to permanent alterations. Now we are talking about the need to decide on the set of functions that will be in the project, and then everything else will go as refinement and support.
The essence of the question is this. How to formalize all the necessary requirements so that it is clear to both developers and designers what and how to do? The project is large and there are many functions. Kanban is already being used in development. Advise on how to organize projects.