In SDDM 4.0 EA1, this problem is fixed. A lot of improvements between the latest version of 3.x and 4.0. The DDL generator and the way if formats as well as solving the DDL dependency problems is a huge help! The added functionality in the object model and how we can use domains for parameters and .dot notation for key generation finally provides the functionality I have been waiting for a number of years. Finally, the diagrammers and the new panel that shows the bird's eye view of where you are on the diagram was a very nice surprise and a huge help when working with large diagrams.
Thanks to you and all of the SDDM team!