your Elemental DMG isn’t affected by Effective Mythic. Effective doesn’t change your Element. it just causes any attack you do to be considered the Element your enemy is weak against, which gives you +25% DMG against all enemies. but because an enemies Natural Element Resist, Greatly Resist, or Immune comes into play, you might not do as much damage as you might think. you can use Weaken with Effective to help. but don’t use Ignore Resist, as Effective and Weaken don’t work with it.
Effective might be good for an Early or Mid Game Build, but there are better ways to do DMG with Late Game Builds.
read this to get a better understanding of Effective, Weaken, & Ignore Resist.