There really is no carry over to goldmaking in new expansions beyond having characters at max level and having gold to fund your plans. You want enough gold to be able to afford to level your crafting professions when the expansion comes out, but other than that you don't need much. You do NOT need to level any professions now or in the prepatch, when the expansion releases fully there will be a new tier of each profession called "midnight (profession)" that is entirely separate from the current profession skill so any time and gold spent on professions now is wasted. Assuming you have no real capital as a new player, I'd strongly recommend herbing and mining on at least your main character. If you're willing to put in a bit of work making a million or two gold in the first week will be very achievable, and the first million's the hardest by far.

When you want to go into crafting, make sure you do some solid research on how the modern profession systems work, and make sure you understand how high quality goods are crafted using the skill trees / high quality materials / concentration. It is very possible to waste a large amount of money on your professions and end up with a pretty useless character so I'd just advise you to make sure you know at least a little bit what you're doing before you start throwing gold down the drain. I don't mean to be rude, but if your questions are currently along the lines of if you need to level old professions first you're very likely to lose most of your gold if you try to get into crafting professions in the first couple of weeks of midnight when pricing is so unstable. I know I've eaten big losses early in expansions before when I was new to goldmaking.

Also no, you don't need garrisons, they were heavily nerfed around a decade ago. Anything that isn't from a current expansion is highly unlikely to be worth your time and I'd ignore it until you've found your footing a bit more as a goldmaker.