full cooldawn

No offense here, but this build is massively missing pen at the minimum. Only has 10 flat, and no % pen from an item like Obsidian Shard, Rod of Tahuti, etc. Damage is going to suffer.

Common companion to Book of Thoth right now is Book of the Dead to provide good power and a measure of survivability.

The build ends up overcapping on CDR when you upgrade to Bumba's Hammer, which honestly isn't suggested for Kukulkan. You really should be going for Sands of Time -> Pendulum of Ages or Conduit Gem into either upgrade depending on what build direction you choose. If you prefer higher CDR via items like Chronos' Pendant like you're showing, you might consider the Gem of Focus upgrade, or just for ult purposes, you might just go Archmage's Gem.

The build probably doesn't need 2 lifesteal items. You could either change Spear of Desolation to Spear of the Magus and drop both Soul Gem and Polynomicon, though I understand the function of Poly on Kukulkan. I would still probably drop SG, again partially because you're still lacking in % pen.