While the menu is usable the way it is, it could be better. If you take a look at the example menu in the best practices link that I provided in the original post, you will see what I mean. Some of the differences include the following:

• For any item in your menu that is a link and not a submenu, the item is read by NVDA saying "collapsed" which is unnecessary and could get frustrating to a screen reader user. This doesn't happen with the best practices menu.
• Using the keyboard left and right buttons move the focus to the next adjacent menu item, but it doesn't set focus to the top menu item and read it. Instead, it just says "Menu". The best practices will focus on the top menu item when moving left or right.
• The escape key isn't being used to cancel out of the dropdown on the menu. A user should be able to click ESC and the menu should close, with focus set back to the parent menu item. This presents a navigational issue for those submenu items that have further submenus. In order for a user to navigate to an adjacent menu, they need to traverse to the left all the way down the submenus before it will move on to the next adjacent top item. With the ESC key, they can move back to the parent item, and then navigate right or left from there.
• Tabbing out of the menu when the focus is on the submenu, messes up the tab order, so the shift tab doesn't navigate back to the menu. If the focus is on one of the top menu items, then tabbing out and back works. However, it should set the focus back to the last top menu item that has focus, not the first item. So if the 3rd item had focus, then tabbing out and then shift-tabbing back should set the focus back to the 3rd item, not the first item. And if you tab out from a submenu item from the 2nd parent item, then shift-tab should bring them back to the 2nd parent item.

Best practices example: https://www.w3.org/TR/wai-aria-practices-1.1/examples/menubar/menubar-1/menubar-1.html