As late as possible. Early on, you’ll want to follow the missions in Battle for Azeroth right up until you get the option to send a follower on a two-hour long quest. As soon as you get there, stop doing the War Campaign missions. You can come back to them once you have reached something like 119 and a half and finish leveling with those if you want. If you don’t use these missions to cap off your leveling journey, then they’re the first thing you’ll want to do once you reach max level.
Last I wanna share a curious bug that almost freezes my game and force me to disable the addon. Anytime I add a (Citadel Crossbow [of the Fireflash]) my fps go from 75 to 22, and It's on a paladin, paladins dont even use crossbows. I noticed everytime I put something in my bags the addon takes the math and my fps goes from 75 to ~22 for a while (3 to 7 seconds) and is think it's internal and also think it's fine.
Anyway, there are no more shortcuts. There are no more "Tricks". It's just straight questing now since Blizzard nerfed other methods of leveling. You can still do dungeon spam, it just won't be as fast. You can still do BGs, it just won't be as fast. Just about the only alternative way to level that rivals questing is Pet Battles(which makes ZERO sense).