This isn't really a tribal-specific suggestion, but Cloudstone Curio or anything to get more ETBs out of Tazri and other allies like Conjurer's Closet
Secret Arcade, Nylea's Colossus, Thoughtrender Lamia, Ondu Spiritdancer, and Sphere of Safety give you huge advantages with tons of +1/+1 counters, forced discard, or the hinderance of attacks to name a few
Your goal is to leverage The Ur-Dragon's powerful Eminence ability right from the command zone, making all of your dragons cheaper to cast
The most fearsome builds of this commander Ive encountered lean heavily into artifact ramp and black board wipes like Toxic Deluge and Damnation to control the board until it overwhelms its opponents with the monsters of the deep
Enchantments The enchantments support the mechanics already touched on with the creatures: Shiny Impetus creates a goaded creature and generates treasure tokens, the Warlock Class makes opponents lose life points as well as draw cards for ourselves and the Dead Mans Chest is in a similar vein as Gonti and Daredevil