I have just experienced this issue and I believe I know what is causing it! Check your files in your project under "Source Engine Exportables". Make sure there are no duplicate names. What seems to be causing this is having a collection named "ExampleName" and an individual mesh named "ExampleName" that is not a part of a collection. (ExampleName is a placeholder name) This will make Blender Source Tools read them as the same thing when exporting and causes it to freeze because it can't distinguish them. Make sure to rename either the collection or individual file prior to exporting to fix this.