Admittedly, I know exactly nothing about how to build a mill deck in EDH. The coolest card I've ever seen, though, was Crumbling Sanctuary . It forces everyone else to play on your terms, and since it exiles, you can just wreck combo decks. I would check out the budget page for Phenax at EDHREC.

Eater of the dead is great in Phenax in many cases if it able to tap you can possibly take someone out. Definitely worth putting you actual deck list

Hey, Persistent Petitioners with Ashiok, Dream Render and Planar Void make a deadly combination for mill. Ashiok is good because for one loyalty he can exile all your opponents graveyards and you can do this up to 5 times. Mill is risky as a main strategy in multiple player Commander because graveyards are a huge resource for most decks; an extensive of an opponents hand. Exiling graveyards means opponents can't use graveyards as a resource after you mill them. Void exiles any card that's put into a graveyard, this includes your own cards, but this hurts your opponents much more than you because of mill.

You can have any number of Petitioners in your deck and they're two drops with 3 toughness. Tapping four Petitioners makes an opponent mill 12 cards. This is a good backup plan for Phenax who will not always be on the battlefield to mill your opponents. You can tap four Petitioners even if some of the them have summoning sickness which can speed up the mill of your opponents. 20x Petitioners is $20 which is budget, Ashiok is $2 and Void is less than $1.

Command Tower, Sunken Hollow, Darkwater Catacombs, Esper Panorama, Grixis Panorama, Ash Barrens are budget land upgrades to consider. Nephalia Drownyard is repeatable mill from a land that less than $1.

Good luck with your deck.

Scheming Symmetry is great with archive trap.

Sorry meant to add this too. Scheming Symmetry is also just a good card for commander in general. A 1 Mana tutor that you can also use politically in order to help opponents or to find more answers for things. Make deals with it. It is great. So would reccomend even if you aren't using it just for archive trap haha. That is just a nice bonus