Endgame crafting is a multitude of things. And this is NOT just crafting....Gathering is part of crafting as well. So you have to look at them together...not separately. That being said....Crafting and Gathering is incredibly deep.
Example: Just learning how to spiritbond your gear is incredibly complex and different set's of gear have different RNG values on what type of materia ranks you could get.
1. Crafting Scrips and Gathering Scrips...Crafting scrips cannot be done if nobody gathers the mats. Period. Most crafted raid food requires scrip farming. These mats can be sold on the MB One just has to simply look up Garlean Cheese and see how to make that...it's work.
2. Tomestone Mats. You can't get tomestones efficiently unless you do your dailies for dungeons/raids/MSQ/PvP/etc... Tomestone mats are required for mid level mats. These mats can be sold on the MB
3. Mid-Level Mats. These are made primarily from legendary node mats and tomestone mats. These are usually the most cost effective way to make Gil. You can HQ them or NQ them...either way they WILL sell. These are required for the 610 gear
4. Materia melding/Penta melding. This materia is gained by scrips, quicksynth spirit bonding, and hunts. Materia isn't required, but it helps GREATLY. Materia can be sold on the MB AND..the only way that you are going farm the new legendary mats is with a whooping 3700 perception. You'll more then likely need food with some materia melding to get to that lofty amount.
5. Legendary Gathering Nodes. These are required for midlevel mats. i.e. Bayberry for Bayberry Cloth, Harkari nuts for Titanboa Leather, etc. These mats are required for 610 gear. These mats can be sold on the MB
6. Ephemeral Gathering Nodes. These are required for most pots and medicines, and some other crafted mats. for Endstone Sands, Eventide Sands, Moon sand, etc. These mats can be sold on the MB
Short answer...if you WANT to make money....follow the REQUIRED points above as NEARLY EVERYTHING is connected to each other.
Crafting/Gather Endgame isn't about making boat loads of GIL...it's about mastering every system and managing your time to be totally self sufficient while dominating the MB. You want that in game house? Master crafting and gather and you'll be rolling in coin without even realizing it!
One also has to understand simple economics as well...buy low sell high...supply and demand....want or need....feast or famine....
Example
Crystals: Buy low, what's the supply?, I need them for crafting, is there a famine of crystals. Maybe I can capitalize on the low supply of crystals since there is a famine since these are needed for crafting!
I honestly get lost for DAYS on the MB researching my next big endeavor for corning a market, or recruiting a partner to dominate a market together. Last time a friend and myself cornered a market it was blue crabs in Shadowbringers. We caught roughly 400 blue crabs each and flooded the market for 10k a pop...zero competition except for ourselves. Anytime someone would undercut us...we just bought there blue crabs....and put it right back on the MB.... we made like 4-5mil a piece. Either way....it's ALOT of work and sometimes you need to be clever.