Access the altar of spellmaking and enchanting first, you'll need magetallow candles from frostcrag spire upgrades. Visit mystic emporium for soul gems, grab fortify attribute spells at skingrad's great chapel, and get drain magicka from the mages guild.

Create a custom spell combining drain magicka (1 sec) with fortify magicka (2 sec), then enchant a ring or necklace with fortify magicka. Cast the spell repeatedly to stack magicka bonuses your magicka will eventually go negative but don't worry. Equip the enchanted item to fix it, and you'll notice your max magicka is now significantly higher. Keep casting to push it up to 9000+ if you want, just be careful since really high values might cause save issues.

Tip: you can duplicate the enchanted item using inventory glitches if you're worried about losing it but the base glitch works fine with just one piece of gear honestly. Makes those high-level spells basically free to cast and you won't need to worry about magicka costs anymore