​How much effort are you willing to put in? The days of clicking the mission table and collecting a big pile of gold a few hours later are behind us. Our gold-making options for this expansion are pretty much just farming (materials or rare drops), flipping, and professions. All of those things are a time investment. If you're not willing to do any of those things, you're probably not going to increase your income.
WOTLK and Cata were my goblin training grounds as well (I remember fondly making 30k on a single day after a patch day with Inscription), but since Im only at Legion levels, Im struggling to find oportunities where to start invest and make profit. I've been carefully paying attention to the market and picking small batches of mats to flip. After 6 yrs out im slowly starting to pick up (so far i think i made 100k in a couple of months).

This expansion suffers from the Warlords of Draenor (WoD) effect, initially, players will think this is a great expansion but upon investing some time into the game they'll realize this game is a disaster. The pre-patch should have been our first warning with how poorly designed and executed that fiasco was, but beyond that, let's look at what the expansion itself offers:

Thing is, as in real life, opportunity cost is the real key on this. I'm getting 2 tokens per month quite easily with just 3 toons (1 maxed only (yeah, pretty damn inefficient but I lack a lot of time to play :p)) just by flipping couple of things, food, flask, potions, bags, shirts (yeah, shirts!!), etc etc. Every damn penny/gold counts :p, just have to learn how your realm market works, what niches exist, which items are needed and undersupplied and which are oversupplied.
So much new stuff. The war questline expands. Island expeditions get a couple of new locations to explore. Azerite armor gets tweaked so it’s slightly less stupid than it already is. While we wait for the servers to go back up, Blizzard’s prepared a handy survival guide for Tides of Vengeance. I suggest looking it over, or else risk not surviving at all.

DMF Cards the first ~72 hours of the expansion. Every deck was selling for 500k-750k after 24 hours, so I spent all the time I had and just shuffled bracers info expulsum. Constantly spammed trade that I was buying herbs at just about every price, and just went to town. Once the weekend hit (Friday), I sold my last few decks (made 44 total in those 3 days I think), and listed all my leftover cards on the AH at a decent cut.
I think that's server dependant. I make a lot of Golems, but the price on my server is high enough that you could buy all the living steel and still make about 20K (almost 200kg on AH). Its just that you proc all the way up the chain. You proc 20% of your trillium, and then 20% of your living steel. SoH and Ghost Iron are such low effort farms, too. 20% of 180 Trillium Bars at 500g is 18k more profit, 20% of 150 Living Steel at 3500g is another 105kg. That's an extra 25k profit per Golem. If they stop selling stockpile them for the next expansion, they always boom with the price of herbs. And a consistent 20% proc rate is the opposite of RNG, its extra free stuff as a benefit to completing an extra quest (not to mention for new alchemists the investment of leveling Vanilla Alchemy). You're looking for some get rich quick scheme, where the real way to build wealth is patience and volume. No market is going to be viable because you want it to be. Only because you make it viable.