Hey, I enchanted ring, amulet, heavy chest piece (dragonbone), heavy helmet (dragonbon) with 4x28% reduction for Destruction and Conjuration. I did the same with only 4x25% reduction in cost for Destruction only.

Both gear sets do not make spells cost 0 mana, just reduce the cost heavily. Some of the spell cost 0 mana (Novice level ones), some cost 3-5 mana (Adept level ones).

As far as mods I use are concerned, its:

spell combos
immersiver armors
dual wield parrying
spike
rich merchants
noautoskillbooks

DLCs - Dawnguard

I tried it with/without the mods, same result.