Yes, Dart or Flutter can target any Desktop with Windows recently joined. I think Linux is Qt based, but not sure about Windows rendering which is either built by Microsoft or a WebView.

Yes, Julia is a great choice for HPC and research. I was biased due to being more of a product engineer where having a unified pipeline (all swift or python) is sweet. That being said, if there is intensive research or HPC requirements in the project, building things in Julia and export to ONNX is absolutely a great option.