"The way the glyph market works on most realms is that there are a couple of people willing to put in the long hours who will have every available glyph posted profitably, and undercut within minutes of being undercut. They all reduce the price very minimally when undercutting, and the "competition" isn't about price, it's about who can relist more frequently.
Welp, not clear on what to do or what to invest in, but (to me) the Key point professions will be alchemy, inscription and blacksmithing, Alchemy raiders will need flasks and pots, so steady income. Inscription for cosmetic glyphs and trinkets the first two tiers of expansion. Then there is blacksmithing that can make ilvl 300 weapons (and armor) that are lvl 110, pretty much like WoD weapons, so they may sell very well through all expansion.
Professions are skills used to create and enhance items (gears). You will start with one skill once your character has learned a profession. When you use the profession, your skill will increase. A profession’s higher skill will allow you to create more powerful items. If you change a primary profession, you will lose any skill you have made with your original profession. So it is important to choose your primary professions wisely.
“For those of you who played Legion, the process will largely be familiar, though in Battle for Azeroth we’re trying to align the opening of a new raid tier with the start of new PvP and Mythic+ seasons, to avoid some of the awkward item-level leapfrogging that we saw during Legion (e.g. the Mythic Keystone cap repeatedly going up to 15 and then back down to 10).”, the game director writes.
The expansion is set immediately after the events of Legion.[4] Two continents have been added within the Great Sea between the Eastern Kingdoms and Kalimdor: Kul Tiras, one of the major human kingdoms, and Zandalar, the homeland of Azeroth's trolls.[4] While the Alliance and Horde will initially travel to one continent respectively, both continents will be available to both factions at level 120.[4]
"Since [patch] 7.3.5 we’re happy with the overall pace of leveling, he said. "If you’ve chosen to start a brand new character right at the end of an expansion, getting a character fully leveled up and geared up to complete endgame content before the expansion ends, there’s always going to be some time pressure there. That’s just part of the nature of choosing to do that at that stage in time."
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Honestly TL:DR of any guide. Is learn your markets, it's as simple as that. Learn who's in it, learn what's profitable, watch different time tables. I've earned 2 million profit OFF 1 market and half set up TSM. I've barely tried before this I was a hardcore raider, only made gold over the years through BOE farming and run sales (which are goood, don't get me wrong) but I want to be making the multiple gold capped characters.

Sky golem is all profit if you do the smart thing and run old panda farms on multiple toons. Was running 10 farms at one point till bfa launched and was upto my eyeballs in trillium and Ghost iron. Just not been doing it lately, but those 10 farms was an hour a day if that and sustained 4 crafters and alchemists though could have sustained 8 easily.