Whatever theme you decide to go with, I have a few rules I follow when brewing Commander decks.

35-50 cards "on theme". So for my Zombie Wilhelt deck, i tried to aim for cards that are zombies, buff zombies, let me tutor for Zombies, give payoffs when Zombies ETB or die, etc.

40-45 mana sources. Lands + mana ramp cards like signets, Sol Ring, talismans, etc. as well as instants/sorceries that can give mana. I usually land between 32-37 lands and 5-10 mana rocks, depending upon deck type and CMC for the Commander.

6-10 Card draw/deck filtering spells.

6-10 "spot removal" this could be something like Murder or counterspells like Disallow

2-4 Board Wipes

Fill the rest with combo pieces or bombs that are game winning or will tilt the game firmly your way.

These are kind of loose rules, and individual cards may fill more than one role. Like Midnight Reaper. It's a Zombie, so it's on theme, but it's also a card draw engine. Or something like Mind Stone that is a mana ramp card, but can also be sacrificed to draw a card later in the game if i'm desperate.

Demonic Tutor Grim Tutor Vampiric Tutor Mystical Tutor Entomb

Those would be the tutors I recommend.

If you want to stick with the mill theme, i'd recommend going to edhrec.com and putting in Gisa & Geralf as your CMDR and then you can choose the style of deck (Mill, Zombie, etc.) They will give you many more recommendations for the best cards to add. A few that I run with a different mill themed deck are:

Mesmeric Orb Mindcrank Maddening Cacophony

I personally think The Scarab God is a better Commander for a Mill themed deck because his activated ability can pull a creature from anyone's graveyard. So you can mill the crap out of yourself (and others) then pull their best creatures out of their graveyards

Wilhelt or Scarab God are probably better commanders for this list.

Phyrexian Altar should be on there 100%. Will give you infinite death/ETB triggers when you loop it with Gravecrawler. Game over if you have a payoff like Blood Artist out.

There are plenty more Blue/Black lands and you're missing most of the tutors as well, depending on your budget and desired power level

Cuts IMO -

Colossal Plow Deluxe Dragster Imperial Recovery Unit Arcanist's Owl Prodigy's Prototype Raff Capashen

I run 30 lands in my Shorikai deck and mana is usually not an issue. Consider more Blue/White combo lands instead of basics, even if they enter tapped.

Consider the following additions:

Crucible of Worlds - you're going to discard plenty with this commander. Playing lands from your graveyard is almost an auto-include for me.

Isochron Scepter - Ability to go infinite mana with Dramatic Reversal and the right mana artifacts on board

Azorius Signet - more ramp

Skullclamp - Auto include because you can sac off pilots for more card draw

Thought Vessel - ramp and no max hand size if you're drawing like you should with this commander