Since the dm package uses DiagrammeR to draw its diagrams and DiagrammeR can use the mermaid-js package which can draw more complex Crow's Foot Notation ERDs:
https://mermaid.js.org/syntax/entityRelationshipDiagram.html
and which DiagrammeR can currently incorporate through this update:
rich-iannone/DiagrammeR#475
it would be great if in the future the dm package could include an option to draw more complex Crow's Foot Notation ERDs using the DiagrammeR mermaid() function.
Thanks! :-)