You’ll have to ensure your script does its work in a thread and doesn’t block Rhino. Regular script execution will block Rhino, which is what you are seeing.
Not sure how easy this is to do with a Python script, I would personally go for a C# plug-in to do the reading and data updating.
Or use something like Firefly (not sure if that works in Rhino 8). See for instance this post: Grasshopper and Arduino real-time communication