enabling sprint cadence with creastive designers

Product Backlog refinement ought to include enough design to allow each item to be understood and estimated.

I am struggling with the need for designers, who are unable to commit to time / story points 

Sometimes the concept of story points is another hurdle that affects transformation and adoption.

I have had success explaining things the following way.

There is a vast amount of scientific proof that shows people can effectively categorize items in no more than 5 categories.   Think of a survey question where you ranked your response from 1 to 5, and how that was different from a similar question asking you to rank your response from 1 to 10.   How do you differentiate what is a 6 in comparison to a 7?

Story points work in a similar fashion, in that teams will most likely self-restrict their estimates into a band of 5 values (1-2-3-5-8, or 2-3-5-8-13, or 3-5-8-13-21, etc.).   Try to explain that what they're doing is simply placing items in one of 5 boxes, from extra-small to X-large.   Also, advise them that they are only estimates, which are guesses based on the current information at hand, and are subject to change based on additional information gained all the way up until sprint planning.

Then, engage them in a quick exercise to simply place design items in one of 5 categories (x-small, small, medium, large, X-Large).   As the team executes several sprints, have them revisit past estimates for consistency and accuracy, and adjust as needed.

Good luck!

Thanks Timothy