Moving the menus to a different file is trivial: your form is declared as
public partial class MyFormName : Form
So you just add more files that also generate
partial
class code for the same class - it's pretty standard for large classes to break it up in this way.
Can I make a suggestion on your menus, that you may not have thought about?
Create a menu for the first scenario, and make it complete for that scenario. Then set it's Visible property to false.
Do the same for the other nine. When you run your code, none of them will appear.
So all you have to do is set the appropriate Menu item Visible = true, and it and only it will appear to the user. Any common items have different names, but the same text, and are "wired" to teh same event handler method. Simples!