Yo! So first things first, when I'm looking at your list, I still see a TON of lands. 40 is A LOT, and we don't want to risk consistently mana screwing ourselves. The average Scarab God deck (per EDHrec) has 36 lands, which I'll make my target for the suggested changes that I'll post below.
Every deck is different and has different needs, but I like my decks to sit at ~34 lands, and anywhere from 6 to 10 pieces of ramp. One thing that I immediately noticed about your Mana Ramp Section is that most of it weren't actually ramp.
Myriad Landscape is, and
Crypt of Agadeem and
Cabal Stronghold tap for something AND have late-game payoff (all good things!).
But when we get to things like
Coral Atoll,
Everglades,
Ghost Town, and
Gateway Plaza, they're all lands, which in most cases aren't ramp. We're in Dimir, and that means we can't play more than one land a turn. So if our "ramp" is just our land base, we're in trouble. Another thing I want to point out regarding
Coral Atoll and the like is that they greatly stunt our tempo in-game. Normally, we want to be playing spells on curve, i.e., it is turn 5, I want to cast a spell that costs 5 mana (like our commander). With lands that enter tapped (like
Jwar Isle Refuge), we put ourselves behind a turn for flexibility later in the game. With things like
Coral Atoll, we're putting ourselves a whopping two turns behind curve, because the land enters tapped and has us bounce an UNTAPPED
Island to our hand. Unless our lands are insanely powerful, we don't want to be paying mana into playing them. That's what spells are for.
My next point is that I'm a bit stingy when it comes to lands entering the battlefield tapped, but I do recognize their importance. That said, I'd like to keep it as low as we can for the sake of our deck's tempo. And as such, there are a few lands that we want to add into your deck that taps for two colors and enter untapped. Those being
Tainted Isle,
Watery Grave, and
Sunken Ruins. If I'm not mistaken, you own at least two of them. And these lands will replace things like
Dismal Backwater,
Jwar Isle Refuge,
Salt Marsh, etc. Another thing to note is that 80% of your mana distribution is black, and 20% of it is Blue. So we don't necessarily NEED all of these dual lands and should index slightly more towards black, as that's what most of your cards will need to be cast.
I'm also suggesting that you add
Evolving Wilds and
Terramorphic Expanse because they offer flexibility in color fixing. They also have nice upgrades when we want to sink more money into our deck (see
Prismatic Vista,
Fabled Passage, and
Polluted Delta, or the other fetches).
Now for some legitimate ramp and color fixing options!
Sword of the Animist is one of the best ramp cards that non-green decks have access to. You're running a deck with a fair amount of creatures in it, and you will probably want to attack with those creatures. Why not make one a little bigger and get a basic land every time you declare it as an attacker?! Wayfarers Bauble is a good way to get land into our hand, similar to how
Evolving Wilds does it.
Solemn Simulacrum is a fantastic card that ramps us for one and draws us a card when it dies (plus, we have options for creatures in our graveyard). The Signets are classics, and if you want more rocks, later on, there are a few more that we can choose from, but for now, I'd definitely get a
Dimir Signet and
Arcane Signet. And last, but certainly not least, we have
Dreamscape Artist. This guy is
Harrow on a stick (IN BLUE no less!). He also has us discarding, which isn't a problem because we can discard a creature for our Scarab God to resurrect later.
Okay, we're in the home stretch, and this is where I will give you some agency to remove cards. Keep in mind this is your deck, so you can ignore any or all of my suggestions, but hey, I'm just here trying to help a friend out. If you've been keeping track, I've suggested you add more lands, so now we have to find the lands to cut. I don't think that you need both
Arcane Lighthouse and
Detection Tower, and would drop one. Most people prefer
Arcane Lighthouse because it removes shroud. I don't love
Dimir Aqueduct, or any bounce land for that matter (they aren't my cup of tea). But it's still strictly better than
Coral Atoll, so if you want to keep it around, feel free to do so. I don't think the Thriving Lands are necessary for this deck, so I would remove them.
Salt Marsh,
Jwar Isle Refuge, and
Dismal Backwater are all meh. The one life gain is rarely going to make a difference in-game, and
Salt Marsh gets outclassed by
Tainted Isle and the likes. I still feel like the deck is a little heavy on the tapped lands (~10 after my changes). And we need one more card cut; maybe it's
Temple of Deceit; maybe it's
Memorial to Folly; maybe it's
Barren Moor. I'll let you decide.
And while my target was 36 lands, I hit my good ol' 34. Funny how that works, huh? If you weren't keeping track of the cards that I would remove and add, good news, I listed them. If the deck still feels clunky after these changes, consider removing more tapped lands for basics, indexing slightly more towards black. Sorry for the essay lol. Let me know what you're thoughts on all of this are.
Cards to Add (11):
Lands (5):
Tainted Isle,
Watery Grave,
Sunken Ruins,
Evolving Wilds,
Terramorphic ExpanseRamp (6):
Sword of the Animist, Wayfarers Bauble,
Solemn Simulacrum,
Dimir Signet,
Arcane Signet,
Dreamscape ArtistThings to Remove (10+1):
Lands:
Coral Atoll,
Everglades,
Gateway Plaza,
Ghost Town,
Arcane Lighthouse OR
Detection Tower,
Thriving Isle,
Thriving Moor,
Salt Marsh,
Jwar Isle Refuge,
Dismal Backwater, {one more of your choice}