kanban Questions

I would like to get some expert opinion on the following product environment in terms of story writing and story slicing. Suppose if we have an enterprise product and many clients would be onboarding to the product in different timelines of deadlines with customized extra features than the basic product. As many clients are getting onboarded with more unique customized features and the overlap is in place, the Product owners are getting challenges in writing the story which enriches the base product that fits our system and slicing the stories into smaller units that fit into the 2-week sprint for the clients. What would be your suggestion or approaches on these sort of situations to address the story writing and slicing challenges?