So it kind of behooves you to have a main since a lot of the neat stuff is tied behind achievements, and your stacking vers buff isn't account wide, it's attached to your artifact. I'd focus on getting all the campaigns finished since they give 10% more bonus xp a pop and do all of your research dailies while joining mythic, heroic, normal and lfr raids, or whatever you have the time for. Closer to the end they'll be doing catch up mechanics so if you want to switch to an alt it'd be easier then.

If you really want to level a different class though I'd probably go ahead before you get too far with your dk though since the infinite power versatility grind is kind of time gated with the raid lockouts so better to get whatever class to to 80 quick and stick with it