I disagree with the balance assessment there. First, to use this you're not using one of the other features, making actually casting shield a better choice most of the time. Second, it's explicitly worse than the shield spell for the majority of the career. Most characters never see 13th level (according to WotC data indicating most campaigns end before 7th level spells come into play).
If you're that concerned about preserving the ascendancy of the Shield spell, then you could have it function like a parry rather than an AC buff, adding prof to AC for the triggering attack only. Still very good, just not as good when you're being focus fired.
Good luck either way!