I find the enchantment slot mechanic rather blunt. You will just max out on enchantments at all times.

I'd like a more dynamic system with a trade-off that would force the player to make meaningful choices and be smart about their enchantments. General rule: if you don't need it, don't use it.

My suggestion: reserve Essence for spell upkeep.

Say a protective enchantment would cost 1 Essence to maintain. While it's on, your effective Essence is one point lower. When you cancel the enchantment you get the Essence back. The more you stack enchantments the more you pay in spellcasting potential. If you want to cast something powerful to the limit you need to cancel all enchantments to be able to do it. So basically it would be a good idea to only have enchantments you really need at the time instead of just stacking them with little consideration.