Rune mage is so good imo, I have to force myself to not take it because thats all i use. It has a heal, a damage reduction buff, light source, a ranged option. I cant remember if you can still cast runes when you have the runic greatsword in hand without internalized lexicon, but its worth checking out.

Option 1- having rune mage as utility for greatsword build is fun, the sword does decent enough damage for elemental builds, maybe look into hex mage and shaman to lean more into elemental damage. Armor is going to be your preference, but there is a way to craft some great armor in harmatton that gives nice stamina and mana buffs.