This is my attempt at a focused, effective amass deck that doesn't have to have all of its lands come in tapped just to be able to play Nicol Bolas, Dragon-God. :)

The landbase can be upgraded with shocks (Watery Grave; Blood Crypt) and a couple scry lands (Temple of Epiphany) as necessary.

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My overall observation on the Amass cards from War of the Spark is that while the mechanic is in the Grixis colours, there is a clear division between the Rakdos side and the Dimir side. One wants to attack aggressively while clearing away all blockers, and the other wants to protect itself while building up an advantage.

This can be seen in the rares from the set:

Rakdos gets Dreadhorde Butcher and Widespread Brutality, a hyper aggressive two-drop that incentivises you to attack every turn and a cheap boardwipe that presses the advantage if you have a large attacking Army; and Dimir gets Soul Diviner and Enter the God-Eternals, a defensively-statted two-drop that incentivises you /not/ to attack each turn so you can draw cards and a high-cost removal spell that slows your opponent down and helps to keep you alive.

Having looked at several of the other Amass decks people have submitted on this site, my main takeaway is that most of them aren't built with this in mind. The author will happily play Grim Initiate alongside Soul Diviner, or with Gleaming Overseer and Widespread Brutality together in the same deck even though the two serve to work against each other. The cards are seemingly only included because they all have 'Amass' on them.

Another thing is the manabase in these decks. Having cards like BBR Bedevil, 1UU Narset, Parter of Veils, 1BRR Widespread Brutality, and then 2UUB Enter the God-Eternals all trying to fit into the same deck is just the epitome of greed, and it's no surprise that some of the manabases are just all of the dual lands slapped together with a hope and a prayer.

What I've tried to do is focus on a handful of powerful, synergistic cards with an eye to keeping the mana base simple and straightforward with as many basic lands as is reasonable. This means there's 20 black sources, 15 blue sources, and 11 red sources with no colourless lands, and there's 8 basics that can come in untapped.

With regards to the cards, the basics are:

Disfigure - This is just some cheap removal that allows you to still defend yourself on the early turns while you're playing out tap lands.

Thought Erasure - This is just too good a card not to be included in any blue/black deck, unfortunate as that is. There's only two though because this deck wants to be a bit more proactive with most of its turn two plays, and running four makes me feel like a bad person.

Bedevil - This is just a terrific removal spell. Being able to hit both creatures and planeswalkers at instant speed is just too valuable not to want to play all four.

Nicol Bolas, Dragon-God - There's no particular synergy with this card, it's simply a powerful planeswalker that can steadily rip the game away from opponents as you plus him behind a wall of creatures, or just get rid of an annoying creature or planeswalker for you.

The cards that are uniquely Amass-related:

Dreadhorde Invasion - This seems like it has to be the core of any Amass deck, and then you choose how to build around it. It provides a constant source of board presence against control, and a chump blocker every turn against aggro. It does synergise with itself by giving 6-power tokens lifelink, but the token dies too easily for that to be your only source of lifegain. This card can definitely get out of hand for your opponent if you start to add a couple Diviners and Overseers to the board, though.

Soul Diviner - This card is great! A 2/3 for 2 stops aggro in their tracks, and a two-drop that can start drawing cards every turn starting on turn 3 (after Overseer or Edge) is gas. An uncontested Soul Diviner alongside Invasion can be pretty disgusting, and even just playing Nicol Bolas, minusing and then eating the last loyalty counter off of him when your opponent is forced to waste their turn attacking him is great too.

Bleeding Edge - This card... a lot of the decks here don't even include it, but I think this has to be an automatic four-of alongside Invasion in any Amass deck. Note that it says "Up to", so you can just play this to make a 2/2 or to boost your 4/4 Army up to a 6/6 out of nowhere for Invasion's trigger, meaning this card is never dead even in creatureless control matchups. Against aggro it's a 3-mana Ravenous Chupacabra, which is a good rate, but just in general it's a great card to pick off your opponent's small things while adding to the board or making your hexproof Army even bigger.

Gleaming Overseer - There are four 'Army token lords' from WAR, creatures that each amass N and then give a static bonus to your Army token: One that gives it trample, one that gives it flying, one that gives it deathtouch, and then the Overseer who gives it both menace and hexproof. In a standard format that is defined by Teferi, Time Raveler and his -3, Overseer is the only one that is viable to play. It's a great blocker, it protects your Army from removal, and combined with Soul Diviner it lets you set up a board that lets you draw multiple cards per turn while still keeping up the pressure. Great card.

Commence the Endgame - I really like this card, even if it's not the most competitive. It's part card draw, part flash creature, part combat trick, and it's uncounterable, so there's a lot of situations where it's useful. It's expensive, but can be very impactful (in a fun way, unlike Command the Dreadhorde). You wouldn't want to play four copies, but I've been finding I don't want fewer than three because of how much I've enjoyed seeing it in my hand.

Cavalier of Night - This card isn't directly related to Amass, but given that Dreadhorde Invasion spits out a saccable token every turn and Diviner and Overseer both tend to eat removal early on, this card has an incredible amount of synergy. The lifelink is immensely helpful in sustaining Invasion and stabilising against aggro. Having extra removal is just always nice. But I think the best thing about it is that - compared to Enter the God-Eternals - this is a proactive creature that is able to sit alongside an Army as an additional threat and, when the Cavalier dies in a boardwipe, it can bring back Gleaming Overseer and create a fresh new Army token.

One of the biggest issues with Dreadhorde Invasion is that if the token dies, the new one that's created has summoning sickness and can't attack. However, if you create an army token on your opponent's turn it will have lost summoning sickness when you go to your turn. Having your board wiped while you have Cavalier out means you effectively begin your next turn with a hexproof Army token with haste and menace, which can be very deadly if you then use Endgame to put 4-to-8 counters on it. :)