Do not navigate from Facebook.com. Bookmark https://business.facebook.com/latest/inbox or https://business.facebook.com/latest/planner to skip the loading screen.

On desktop, navigate to Settings > Business Assets > People . You can assign "Partial Access" to a graphic designer (so they see only the Planner) and "Full Control" to the marketing director. The desktop UI makes permission toggles far clearer than the mobile app.