Hello everyone!

Is it possible to play on the highest difficulty setting without power gaming?

Yes.

Just be aware that there might be some unexpected effects of different difficulties.

One that I am aware of is that the damage you take when rage ends is affected by the difficulty. If you are playing on Hard or Insane, you will take more than 15 damage when the rage wears off, resulting in a net loss of HP. (On Easy or Normal, by contrast, you don't lose as many HP, resulting in a net gain of HP.)

Also, I believe your summons don't take extra damage on harder difficulties, which makes such spells more useful on harder difficulties (though not to the extreme of Heart of Fury mode in Icewind Dale).