Feln Mill

I rather like this deck. Basically all of your cards that mill serve some sort of secondary purpose, usually keeping you alive. Dreamsnatcher for example provides early sustain and can be a threat in the early game with a Tome of Horrors or late game with its ultimate. Mournful Deathcap has a healthy body and has consistent mill. Two-Face is difficult to swing into and can help keep you alive.

Darktalon Wyvarch and Eremot, the Gathering Dark are evasive and can threaten lethal damage over several turns. The mill is essentially tacked on.

Now the nonmill creatures. Blightmoth is evasive and can hinder Strangers or Grenadin, and sometimes it just slows your opponent's clock. Surprisingly effective card in my experience, it's basically always welcome.

Silverblade Menace is insane in this deck. I regularly hit for 10+ damage and heal as much, it's evasive, it blocks in the air, it heals you, it combos with Tome of Horrors. MVP of the deck for me.

Icaria, First Reaper is a bit of a pet card. I wanted to play with it, it's a high value evasive body that impacts the board immediately. It's essentially good stuff and you should run it if you can afford the 6S influence requirement.

Tome of Horrors is a surprisingly relevant card. This deck has a lot of units, so it mills aggressively. It also provides A LOT of card advantage against slower decks. When I saw it, i thought it was winmore. But it comes down on turn 2 or 3 if you started Dreamsnatcher or Two-Face and enables cards like Essence Feast.

Seek Power is self-explanatory. 99% of the time, you'll want a shadow sigil so you can ideally play Icaria, First Reaper on curve, though occasionally you may need primal for Darktalon Wyvarch, Wisdom of the Elders or Hailstorm

Essence Feast is a 2 mana boardwipe, and the deck's mill plan nearly always enables it. The card requires some thought as it kills your stuff too and you may need to do some math to know how many cards you can mill on the fly. Eremot and Wyvarch are RNG, Tome of Horrors scales with unit damage. The card can also keep you alive with lifesteal depending on how wide each players board is, though 5-7 life is pretty typical.

Strategize and Wisdom of the Elders are card draw and can be played at any stage of the game. They're flex spots, if I were to add a market, I'd make cuts here.

What the deck struggles against

Grodov's Stranger and Lumen Reclaimer can be problematic for you since they can save your opponent's void. If you're seeing a lot of these, consider Royal Decree and Turn to Seed, even Feeding Time if you're desperate. These cards are what I'd start with on a prospective market.

Relic Weapons have been fundamentally a problem for Feln since ye olden days when cards like Champion of Cunning reigned supreme. On the off chance you're against armory, and I mean like true armory with Rakano Artisan, Sword of Icaria, Starsteel Daisho - feln's color identity has trouble dealing with these cards. You have ways of interacting like Burglarize in your colors but they're few and far between. A hypothetical deck running this strategy tends to run a lot of board wipes and several relic weapons, so if you get that read against such a deck - play your units sparingly, try to bait out board wipes with like 2 units and hope you don't get burst down by like a Sword of the Sky King or Icaria amp'd Starsteel Daisho.

Reanimator Decks. Fundamentally speaking, you are putting their units in their graveyard for them, literally playing into their strategy. You can hope they don't draw their enablers i.e. Grasping at Shadows, Vara, Fate-Touched, Stirring Sands, but a turn 5 Azindel, Revealed can be backbreaking for any "fair" deck.

Combo Decks. Recently I've been seeing a deck that generates a ton of power and vomits their hand. If you're milling them and see cards like Bladecrafter and Ruckus Rouser, know that the latter combos with Heirloom Blade and assuming I understand it properly they're about to get +12 power. I've seen it a few times and because I didn't understand what I was playing against, I lost. Most of the time though, combo decks are going to either lose their combo pieces (cards like Talir, Who Sees Beyond, West-Wind Herald) or combo you to death. The deck as-is doesn't have a way to stop most of these strategies besides sending them to the void, so there's more evidence that I need to add a market. Another option is going back to Steward of the Past or even Nullblade if the above strategies are especially common.

In Conclusion

if I were to add a market, I'd probably use Blight Pass Smuggler, cutting Strategize and one Wisdom of the Elders

with the following market cards

Royal Decree
Turn to Seed
Nullblade
Feeding Time
Pale Rider's Timepiece (as an alternate win condition, could be substituted with something like Champion of Cunning if you want to save shiftstone)