UX Navigation Spaghetti

I am revising an older navigation solution. The main nav has six L1 menu items. Then each L1 has multiple L2 items in the mega menu. But the problem I am having is that each L2 item also has a submenu of 4-10 items on the the page. I am thinking there is a better method to do this. This particular site is very heavy on content which justified it. Any ideas?

Maybe get your users/potential users to do a card-sort and tree-jack. exercise