I’d like to jump in on this! I have the same issue. I don’t want the submenu to open when I tab to a main nav item. It should only open when the return key is pressed. If the nav item is both a link and sub menu toggle, a keyboard user should first end up on the link, then on the next tab press focus should end up on a toggle button and the return key should open the sub menu.
All according to W3 that reference the WCAG standard.
https://www.w3.org/WAI/tutorials/menus/flyout/#keyboard-users
