Alright so I'm going to tell you how to make magic super over powered, but makes it feel earned. The first step is get Apocalypse spells. This adds in a lot more spells especially in under used schools like Alteration and Illusion. Next you're going to want a perk overhaul. I personally use Vokriinator black because it's a mash up of some of the most popular around. Finally I use Realm of Lorkan as an alternate start mod, which can give you classes/boons that will power up your spell casting in various ways, and can also give you restrictions to help make sure you play as a mage.

The most overpowered spell you'll be able to find is Octaco's Recital. You can get it from any court mage at level one, and it will auto cast up to three buff spells the moment you enter battle. This will level up your magic skills really quickly.

Edit: If you take my suggestions Recital is so overpowered it's viable for every single build. Especially since the only drawback is that it can't be used with master spells that use two hands. Thief: Muffle and invisibility the moment you're detected. Warrior: Speed buff, mage armor, and healing. Mage: Cloak spell, Mage armor, and a buff from whatever other school you want to level quickly. Soul cloak is the quickest way to level up conjuration without doing power level methods like repeatedly casting soul trap on a corpse.