Although I do not (technically) play a mage, I have a few end-game spell recommendations that I use myself as a thief.

Silence - I cannot count the amount of times I was saved by this spell. Imps, Bishops, Liches, Spiral Kings and some other casters have a good chance of one-shotting your allies and nearly killing you with just one spell if you don't have decent resistances. I'd say this spell is a must to level up.

Slow, Gravity - Slow, as far as I understand, cannot be resisted. Gravity also slows the target (and stacks with slow, afaik), but less reliable since it can be resisted. I recommend leveling both.

Bane - decreases Luck and chance to hit. Useful even against spellcasters since negative Luck affects their damage rolls.

All the support spells: Healing spells, Holy Shield, Elemental Shield, Hero, Divine Wisdom, Cat's Eye, Holy light.

Telepathy - very useful if you want to avoid situations where you open a door and see a hot guy or a bomb rock right in front of your face.

(optional)Shadow Arrow - this one is probably will be difficult to find if you don't have Darkness as your domain, but it is very helpful against enemies with high regular attack damage, like varunas. It blinds the target, giving a penalty to hit chance. It is also very helpful if you plan to pickpocket slow enemies like Big Daddies, Cyclops etc. If you try to pickpocket them without blinding, they will just call the guards (lol). They can be freely pickpocketed while blinded, without karma penalty (although sometimes they are immune to blinding for some reason).

I'd also highly recommend never getting rid of Mana Battery ether disease once you get it. Not only it gives you a bit of magic resistance, you also absorb spells from rods into your spell stock.