This guide assumes that you have read all the previous guides, have arrived at level 130, achieved max kill of at least 18 monsters from masteries with Slayer V, have all T7 E10 or above gear (or working for this goal), maxed 4x mining lasers of an ore and have progressed through the Steamdrake/Deathwhisper part of the main questline. This guide will cover the T8 and T9 equipment and the Maw monsters, focusing on the 4th, 5th and 6th dungeons.
This will be the slowest advancement of the game so far. Nothing compares to the gruelingly long progress towards getting nearly full T8 equipment, which is the eventual goal. With the introduction of Slayer mastery, there is no more set max power limit where getting more powerful past it doesn't matter in the slightest: there is no ceiling to killing more of the most powerful Maw monsters for income anymore, the ceiling is your max kill count.
The focus and main goal of the guide is 2 parts: the first part is getting powerful enough to complete the daily missions for the 4th dungeon and using the resources from it to craft T8 pieces in the order of how easy the core requirements are. The second part is getting powerful enough to actually kill the 6th dungeon monster: gargantuan monstrosity.
The thing that makes T8 extremely lucrative is the incredible increase in base stats and the fact that there is no RNG in the stats. Every piece grants an average of base 60 savagery and fortitude (if distributed evenly) compared to the T7 counterpart. This is purely from reinforcement. Another part is the T8 energizements provide a nice boost, most notably in the IDR energizement in gloves (from 13% to 18%).
The thing that makes T8 extremely expensive is also related to reinforcement. Since you need to reinforce 20 times (T7 was 14) and the fact that mech shards don't drop and need either dungeon attempts or crafting, you need around 600 mech reinforcement shards for every piece except the sidearm. With the current crafting changes of reinforce and energize shard changes (it was monster parts before, now it's gold and lower level of the same item), it has become extremely cheap to make: costing between 40-60M gold for reinforcing any T8 equipment. The mech energizing shards aren't used as much until the later pieces that require very specific energizements like speed, IDR and armor per level, so their cost is small compared to reinforce. It's recommended to craft the first piece where any general energizement can work and do full rerolls until you get it, then adjust your other gear accordingly; this will be the most cost efficient method.
The best place to get the mech shards is the 4th dungeon and the market. The 3rd dungeon is a net negative because of the daily quests requiring too much to get it. The 4th dungeon, even with the reduced price of mech shards, is a net positive and a great source of income. However, don't sell the mech shards; all will be used.
As T9 came out and people moved into it, a lot of decently empowered pieces are on the market, up for grabs via trading with coins. Calculating how much a piece is difficult since empowering and energizing is extremely tricky to calculate. I would suggest calculating how much it takes to craft, reinforce, ballpark an energizing cost, then calculate the E10 cost and then compare with 2 coins + cost the player makes. I have no idea how to value the E13, E14, E15 equipment, so you decide.
The Precipice of the Maw questline provides 340 mech shards of each, 60% of a required T8 piece. Also, the later quests reward some free 4th dungeon runs. It makes life so much easier to complete the questline and get these shards and complete the runs to craft our first T8 piece. However, there are 3 main roadblocks in the way:
The Depth of the Maw questline, up to An Odd Structure I provides 115 mech shards of each. Continuing after that point isn't that profitable, so it's best to stop until you get several T8 pieces. If you are full T7, it's actually impossible to kill the 6th dungeon monster, and don't get into the hell that is killing the later T9 monsters 2-3 at a time.