Generally favored against Lessons, Reanimator, and most flavors of traditional control.

Black Sun cast for zero to wipe lesson tokens.

Flash is your friend!

4C Reanimator
-4 Requiting Hex, -1 Spell Stutter
+1 Disdainful Stroke, +1 Annul, +1 Soul-Guide Lantern, +1 The Unagi of Kyoshi Island, +1 Ghost Vacuum

Bant Airbending Combo
-3 Bitterbloom Bearer, -4 Kaito, Bane of Nightmares
+1 Essence Scatter, +1 Disdainful Stroke, +3 Day of Black Sun,
+2 Vren, the Relentless

Boros Aggro
-4 Bitterbloom Bearer, -2 Deep-Cavern Bat,
+1 Essence Scatter, +3 Day of Black Sun, +2 Vren, the Relentless

Dimir Midrange (Mirror)
-1 Cecil, Dark Knight, -2 Spell Stutter
+1 Essence Scatter, +1 Day of Black Sun, +1 The Unagi of Kyoshi Island

Izzet Lessons- Threat is monument and artist’s talent,
-4 Enduring Curiosity, -2 Shoot the Sheriff, -2 Spell Stutter
+1 Negate, +3 Duress, +1 Annul, +1 Soul-Guide Lantern, +1 The Unagi of Kyoshi Island, +1 Ghost Vacuum

Izzet Looting
-2 Enduring Curiosity, -2 Bitterbloom Bearer, -2 Spell Stutter, -2 Kaito, Bane of Nightmares
+1 Essence Scatter, +3 Day of Black Sun, +1 Annul, +2 Vren, the Relentless, +1 The Unagi of Kyoshi Island

Jeskai Control
-1 Cecil, Dark Knight, -2 Shoot the Sheriff, -4 Requiting Hex
+1 Negate, +1 Disdainful Stroke, + 3 Duress, + 1 Annul, + 1 The Unagi of Kyoshi Island

Mono-Greed Landfall
-2 Bitterbloom Bearer, -3 Flitterwing Nuisance, -2 Spell Stutter
+1 Essence Scatter, +1 Disdainful Stroke, +3 Day of Black Sun,
+2 Vren, the Relentless

Mono-Red Aggro
-4 Bitterbloom Bearer, -2 Deep-Cavern Bat,
+1 Essence Scatter, + 3 Day of Black Sun, + 2 Vren, the Relentless

Selesnya Landfall
-4 Enduring Curiosity, -2 Bitterbloom Bearer, -2 Spell Stutter
+1 Essence Scatter, +1 Disdainful Stroke, + 3 Day of Black Sun,
+1 Annul, + 2 Vren, the Relentless

Simic Ouroboroid
-2 Spell Stutter, -4 Kaito, -1 Bitterbloom
+1 Essence Scatter, +1 Disdainful Stroke, +3 Day of Black Sun, +2 Vren

Sultai Reanimator
-4 Requiting Hex, -1 Spell Stutter
+1 Disdainful Stroke, +1 Annul, +1 Soul-Guide Lantern, +1 The Unagi of Kyoshi Island, +1 Ghost Vacuum

With the meta consisting of cub decks as the main go wide deck, Although the matchup isn't straight and sometimes they have the edge over you, the fact that Dimir gets access to 1 mana answers such as hex and spell snares, really helps to mitigate these go wide boards and potentially buy you time back into the game. I will also state the fact that I run 3 Day of Black Sun in the sideboard to deal with these problems. Usually since you control when the board wipe comes down, it is better for you to lose two 2-drops rather for them to keep their board full of cubs and mana dorks to swing you for the game with ouroboroid/craterhoof. Dealing with their dorks also indirectly deals with their ouroboroid/craterhoof gameplan. Intimidation Tactics is also a nice alternative to deal with a wider variety of threats in their hand.

Against decks like Mono Green Landfall, can't say I've played more than once against that matchup. Sapling Nursery is unfortunately just one of those cards that can really end the game against Dimir if not countered. I would say against these harder matchups, matchup knowledge is really important. You really need to know when to holdback counterspell or keeps hands that have hand hate against these kinds of decks. Which then I think the argument to increase disdainful stroke to 2 becomes better, if you are struggling against these kinds of decks that just play big spells fast, definitely increase it to 2.

One-mana interaction like Annul, Spell Pierce, Duress and etc. now I highly regard, as being able to drop cheap creatures and interact with some core noncreature cards for one mana becomes a huge tempo gain when playing against most izzet variations(lessons, spellementals, midrange, looting, etc.)

Graveyard hate has become more important than I thought, sometimes decks have some weird unexpected synergies through cards like Nature's Rhythm, Superior-Spider Man with Deceit and etc. 1 mainboard strategic betrayal, 1 side and 2 sideboard Soul-Guide Lantern has become a mainstay for me. Unlike in the past where you would answer graveyard decks through other means, having graveyard hate has become very important to stop those strategies from turning the game against you.

Unagi is also a good top end curve, since it can pressure when your opponent doesn't do anything, or help be the last big attacker to get through a stalemate. But that is a one-of in the mainboard, which most people will consider a flexible slot, consisting of anything that more suits your gameplan.

I will say, I personally think Dimir Midrange's inherent strength is to play on your opponent's turn. So my deck is built as so. The most impactful change so far in the 3-drop slot has been Glen Elendra Guardian in the mainboard. Too many Tidebinders and they get no value if you dont stop an ability, likely almost forcing you to flash it out to either pressure your opponent or have an emergency blocker. Plus only having 2 toughness means it can die to a lot of things. Glen Elendra Guardian helps by having evasion, a relevant ability and 3 toughness. Almost guaranteeing your opponent must have 2-mana removal or some sort of premium removal to deal with it permanently.