For future reference, it is a great option to purchase mounts and pets as a long-term investment. Not only do their prices scale well even when a new expansion drops, but there is always a demand for them. The only drawback to this strategy is you would need a hefty amount of initial wow gold investment and also maybe have to go through the motions of spending for a guild bank.
I know if you aren't using hunters than you'll really want the loot-a-rang for easy looting, so that would take up one of your professions. In all honesty If i was going to do it id level up a hunter team and only hyperfarm with them, while doing other things with the mains. As far as regular professions go, they don't really seem worth it anymore like they did back in the old days, so I'd personally stick with gathering stuff as you are hitting nodes x5
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-Bugs- There aren't too many glaring bugs anymore, but you will certainly encounter some. The pathetic thing is that many of the bugs you'll encounter were reported repeatedly during Alpha/Beta and still weren't fixed. Some bugs were discovered after release and players were able to abuse game mechanics without repercussion unless they were one of the more serious offenders. This gave some players a significant advantage over others because they used exploits and at the start of an expansion, that is the time when it's most impactful.
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All I know is that Scribes are going to make bank with those Contracts. All guides talks about them and everyone will want them. Better ride that money wave while we can. Other than that, Alchemy and Jewelcrafting are always safe bets with tradable and marketable goods. Engineers can get an item for followers to bring back more Azerite, ot sure how much of a headstart that's gonna give them. The rest of the professions honestly looks kinda lame since most of everything is BoP anyway.
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).
I believe they risk losing some long-time players forever with the extent of the game-wide squish. Other facets of it are more appropriate for discussion elsewhere, but those profession points represent prior years for plenty of people. Recent xpacs have made skill level less important, which worked fairly well in terms of balancing the past and present.
Sky Golem factory; Use all your spare alts to craft those. Level their Pandaria engineering to 75 by farming ghost iron ore (mostly) and start doing their daily CD's. It's super easy to find informations on how to do it quickly, look it up. 4-5 alts = 4-5 golems a month = a million gold a month if golems go for 200k+ like on my server (250k on mine)

Like I said, engineering is the best when you aren't worried about making gold. The Loot-a-rang is super useful, nitro-boosts are nice when they don't fail, and the glider cape enchant is nice too... you get wormholes to Pandaria, Draenor (make sure you have a glider), Legion, and Argus (as well as a couple old world teleport locations)...Jeeves (and to a lesser extent Reeves) and MOLL-E are also super useful (bank and mail access anywhere, repairs, etc.).
Despite all of these problems and this laundry list of glaring systematic issues, the crowning achievement of Battle for Azeroth’s endgame is that it still remains enjoyable all due to those two pillars of raiding and dungeons. That won’t last forever. As the expansion goes on, and the grind necessary to progress becomes even more demanding and the time spent in content that simply isn’t fun increases, it will be harder and harder for raiding and Mythic+ to carry the expansion along.
-Azerite Gear- This has been the worst experience for many players as it was advertised as offering the player many options for customizing their character but all players do is Google "Icy veins (Spec/Class) azerite ranks" and select the best one. This is expected and really only gives players the illusion of choice as they're always going to pick the best one. This could have been done far better by not offering necessarily just damage increases in traits but offering some of the Legion Legendary item effects in Azerite slots. You know, certain Azerite traits would be better for PvP, some for raiding, some for 5 mans, some for soloing old content, etc. The other problem with Azerite gear is that it's locked based on you grinding an endless amount of Azerite for your neck piece (basically the stuff that increases the level of your neck piece). So when you get an item that should be an upgrade, it really isn't because you don't have all the traits unlocked. This complaint about how horrible this system is could go on forever, but I'll just leave it at that, horrible.
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.