Ah! Dynamo created elements should introduce element binding into the graph, which means you can update the elements by running the same graph a second time (one graph per import per project!). Alternatively you can access the IDs of the created elements for previous runs via the bindings, and export that and the associated ‘Mark’ values to an excel file.
You can then use that excel file to pull the associated elements ID or GUID via the SAP mark.
It’s a pain to reverse engineer but the ‘how to’ can be found in the AU course linked to this thread: Element Binding in Revit
Long term best to create the relationship via exporting the GUID and mark at time of first import (assuming bindings won’t work due to lack of graph/file management).