I’d have crafting on main, precisely for the rep recipe reasons. However, main will probably be out in the world the most as well, so gathering can happen almost on the side. I use one craft and one gathering with synergy on main.
Best to have professions that will be of benefit to your purpose (i.e. if you’ll be raiding or crafting for AH or doing a lot of outdoor content…)
I naively picked Tailoring and Enchanting when I started because I could make my own bags and enchant my own gear but the rest of Tailoring is essentially useless to me, as a Hunter, unless I’m selling items. As I have most of the recipes now, I can’t bring myself to unlearn it.
Mining and Blacksmith are good options (especially if you’re a plate-wearer) but be aware that blacksmithing is notoriously difficult to level!
may I suggest enchanting on your main that’s it’s the one that’s most likely to come across the largest quantity of epic gear that you don’t need
You could also take alchemy for the 2-hour flask buff.
Or engineering for the combat Resurrection
I tend to put on my main enchanting and then a gathering profession the idea being that I’m spending the most amount of time on my main therefore it is the one most likely to benefit from having a passive gathering profession due to the amount of time spent in the world. Then I will level all my alts usually in the following order.
The rogue first who has herbalism and alchemy. Followed by the demon Hunter who uses skinning and leatherworking so I can feed into flips for enchanting. These two basically handle my raid requirements.
Next I will usually level some form of miner my husband has a jewel crafter so I can feed ore into him. I will then normally start off with herbalism on this character as well and when it reaches max level swap it to something like blacksmithing
after that each of my alts are levels with one gathering and one crafting profession. So I will have a herbalist inscription. Mining and engineering. ECT. Once I have all of the professions I need again I ignored JC and tailoring as my husband handles those I will then level characters initially as double gatherers and then swap them to a profession that has a daily cooldown that I can use. For example in BFA I had 6 alchemists doing the anchor weed transmute
Shadowlands however does look to mix up the matter slightly. As personally I now need to get about 3 paladins 2 max level I’ll probably levels and both of double gatherers and just lean on my guildies for a bit of crafting