I would recommend
Servant of the Conduit over
Druid of the Cowl, it will help you hit your double white spells and the black you need for Despair.
The energy will also work well
Aether Hub. Another bonus is it will be easier to activate Rhonas with 2 attack base.
I would recommend going with 4 copies of
Blossoming Defense and 2 of
Hapatra's Mark. I can see the +2/+2 coming in useful more often than the -1/-1 counter removal.
I like the idea of
Cartouche of Strength for removal and to get the trample ability but less dependent white removal would be more consistent.
Something like
Thopter Arrest or Ixalans Binding.
Also, there is a new
angel coming in Dominaria which could fit well in this deck, check out
Lyra Dawnbriger.