Is there any Mega menu module?
You can try the Ultimenu module. It has Drupal 8 version too and it's stable.

https://www.drupal.org/project/ultimenu


  Ultimenu is the UltimatelyDeadSimple™ megamenu ever with dynamic
  region creation.
  
  The module manages the toggle of Ultimenu blocks, regions, and a skins
  library, while leaving the management of block, menu and regions to
  Drupal.