How does it compare to PlantUML with the C4 plugin is the first question coming to mind.
Great question. They are similar concepts in that they both allow you to define a C4 model and generate diagrams based on that model. The key difference is simply that PlantUML (and Structurizr) allow you to define the model using markup or code, whereas Carbide uses a graphical interface. I know some people prefer the code or markup approach, and I know others that prefer a simple graphical interface. I think it comes down to personal preference really, but in the end they accomplish the same thing, which is model-first diagramming.