Like other shields, the secondary ability only triggers when damage actually goes through the shield. E.g. malignant cells would not trigger spike production.
Solar Buckler still produces quanta for creatures with 1 ATK, I think it makes more sense that this shield should do the same.
I think giving it Bonewall-like PC resistance makes it a bit OP. Since the stacks presumably also act similarly, it would basically be unstoppable.
In the long run one Feldspar Aegis = 2 UG. It's slower, but can't realistically be destroyed.
I could go either way on it honestly.
I figured it made sense the way I listed it because the idea is that pieces of the shield are getting broken off during attacks. I.e. if a creature doesn't hit it hard enough, then the piece won't break off.
The UG analogue is intentional. Speed is a big difference though. Also, the opponent has some control over how quickly they race to their own fate. That isn't the case with UG. I think that should be enough to keep the balance, although playtesting will probably be the only way to know for sure.