As far as i know, they havent released any details about the changes theyre making to the battle system as it relates to cross-class abilities. Theyve just said some general nonsense about there being a change, and we've been stuck speculating for months. We should hear actual info soon though, as the expansion is less then 3 months away.

As for crafting and gathering... So having a class at level 50 before you start crafting offers you a couple small advantages, but no big ones. The game gives you bonus exp for any class at a lower level then your highest one, so extra experience is obviously a plus(but not really a direct advantage, just less time spent leveling). And if the item you want to craft has a component that drops from an NPC, being higher level means you can more easily go out and get it. I dont consider that a big advantage because generally the thing youd need to kill would be close in level to what you need to craft the item(if not far lower), and often these drops end up selling for pennies on the market board because their use isnt obvious and they drop often. The advantage to trying to work in tandem is that you could potentially make things you currently or will soon need. With that said, most people who do get into crafting eventually level up all the classes because the cross-class skills in crafting make a world of difference in finishing things easily/at high quality/both. My general advice is to pick a class(for instance armorsmith to potentially make your own gear) and level it to about 15. Generally this can be done in around an hour without leaving the crafting guild, as they have a material salesperson right there. If you like the crafting system, then you can do the same thing for the other 7 classes(level 15 is also where you get your first crafting cross skills, the rest are at 37 50 and 54). If you do go all the way, its a little easier to level them all roughly equally. That way they can share gear, and you can make your own components to save money(for instance, a lot of armor recipes will have some leatherworker components as padding).

Gathering will also help you save money on crafting supplies(or make some money if you decide to sell what you dig up). It works differently though, as there arent really any cross-class skills to worry about(they exist but they arent useful). They can share gear though, and both botany and mining are really easy to get into and get what you need out of. The gathering log tells you roughly where things are, you have skills to pinpoint their exact location, you go there, choose what you want to try to dig up, and get it. Fishing, much like in real life, is its own separate thing. If you need a specific fish you need to know where it lives, what it eats, and when it comes out(only at night? usually when it rains? other even worse scenarios?). Sometimes this is all super easy, sometimes it feels punishingly random. At its basic level, you pick a spot, bait your line, cast, and wait to see what(if anything) you catch. As someone who doesnt enjoy fishing in any form, im probly not the right person to advocate for it.