Here are some ideas

I totally agree that synergies are too strong and should only be like 10-20% of the power of a skill

But also they need a rework

Instead of being specific skills like “firebolt for fireball”

It should be affinites, like “fire skills synergize with fireball”

That open up other class skills as synergies

So your Trang set necro could have its fireball Oskill buffed by necro fire, projectile and aoe skills as fireball has these 3 properties