I’ve never completed the Shadowlands campaign. I wanted to go back and unlock SL mogs. Upon arriving at the main city hub, I’m asked whether I want to unlock my covenant immediately and start Threads of Fate or do the Campaign and THEN pick my covenant. I have no idea what to choose and what will get me renown / mogs faster.
I’ve never done the campaign before so I have no rep tokens or whatever. Very confused and getting mixed info all over the place.
You don’t start the renown grind until you’ve picked a covenant, so skipping the campaign will be the faster option. The main reason to do the campaign would be if you are interested in the story.
Apparently, you can’t switch to threads of fate after picking storyline anymore, despite the fact that the in-game description explicitly states that you can. When I go to the NPC where you switch, it’s not possible to do that.
I wouldn’t have made the choice about which to do if I knew that the one that it says you can change later can actually no longer be changed. 
It’s completely bizarre to me that I have to go back and play through all of Shadowlands on a level 70 when I already did everything I was supposed to back during the expansion when it was current content.
Please someone tell me I’m wrong lol
P.S. Because if this is the way it is now, it’s bizarre that they managed to find a new way to make Shadowlands unlikeable. I spent a lot of time the other day trying to look around online for advice to help me make the choice and never saw anything that said the in-game text about being able to change was wrong!
It’s so awful. I made a thread about it here if you want to lend your voice:
https://us.forums.blizzard.com/en/wow/t/trying-to-get-sl-mogs-is-miserable/