Trigger on a miss i.e. deflection is what I suggested before as well. BG3 could easily even calculate what is a clean miss and what is a deflection, and animate the events as such. You could then have triggers on the actual deflection.
Still, perhaps unnecessarily complicated to explain in a spell description. You would need a new AC value for deflection, like Touch AC used to be in earlier editions. 5e has simplified those things so perhaps it would be better to just have a more straightforward implementation of a 5ft damage aura.
