from my point of view this task is 100% a UX cup of tea.
From my experience (and from the ones coming from colleagues) I learned that, not always (for different reasons), you can have a product development process that includes UX topics since the first phases of the project.
When I’m in this situation, usually, I proceed by applying the MVX (minimum valuable experience) approach. Step by step and release after release I try to increase the overall UX level improving the consistency of the UI, the consistency of the interactions and as much as possible simplifying the processes.
My tip is to prepare an execution plan divided by steps/releases and share it with the project stakeholders. Analyze, evaluate and at the end decide which UX KPIs the project should achieve. This exercise allowas you to have a shared execution plan.
Afterwads it will be easier evaluate it and, in case, change it.
Anyway it’s not an easy task, good luck!