The signets would be good but I think the talismans would be a better fit and can help a bit to lower that life total to get the dethrone ability going.
I like your list and as you've said seems to be a bit on the slow side against heavy aggro decks.
I love shivan harvest in this deck.
Have you thought about these possible cards in the deck.
Thoughtpicker Witch
Oona's Blackgaurd
Puppeteer Clique
Purphorous God of the Forge
Skullclamp
Sol Ring
Attrition
Demonic Tutor
Vampiric Tutor
Toxic Deluge
Diabolic Intent
Phyrexian Reclamation
Chromatic Lantern
Power Conduit
Tymaret, the Murder King
Dominus of Fealty
Insurrection
Stalking Vengeance
Vela the Night Clad
I love shivan harvest in this deck.
Have you thought about these possible cards in the deck.
Thoughtpicker Witch
Oona's Blackgaurd
Puppeteer Clique
Purphorous God of the Forge
Skullclamp
Sol Ring
Attrition
Demonic Tutor
Vampiric Tutor
Toxic Deluge
Diabolic Intent
Phyrexian Reclamation
Chromatic Lantern
Power Conduit
Tymaret, the Murder King
Dominus of Fealty
Insurrection
Stalking Vengeance
Vela the Night Clad