I have encountered similar requirements and met the unfortunate realization that Flow Designer fails miserably in this regard. This is something that works so effortlessly in the now legacy Workflow Editor but it's a foreign concept in Flow Designer. It's as if the development team never intended Flow Designer to be used in real life Service Catalog scenarios. There are some workarounds suggested in Community posts, but I couldn't get any of them to work reliably except this one - create a new field on the sc_req_item table that will be updated instead of the variables, or re-purpose an existing unused field. In either case, the users will either update the field directly, or you can create a Business Rule to update the field when the variable is updated. I know, it stinks. ServiceNow's own documentation says to avoid using Catalog variables in wait for conditions, if you can believe that. I have found this recommendation to be sound - results are inconsistent / unpredictable when executing a flow that waits on a catalog variable. It works, until it doesn't...
