Unfortunately it’s the only way we support, you’ll need to know enough HTML & CSS to recreate your nav in Discourse.
You could potentially go the opposite route (which might be a little simpler) and use a pre-built theme component in Discourse like Brand header theme component… and then match your main site’s nav to Discourse. But then of course if you wanted to customize the look of the nav further than what that theme component contains, you’d need to know some HTML & CSS.