@Porkcarrot, this might help you get that experience with a minor trick;
Find someone who has completed the campaign, specifically the last quest in act 5.
Have them start their campaign mode on that last quest.
Then join their party and follow through that last one quest.
Your account will now be flagged as having completed the campaign, but without having spoiled all the stories for you. Adventure mode is unlocked, and just do one grift to enable doing the weekly challenge rifts etc.
It's not a complete solution to what you want to achieve, but a step in the right direction perhaps...
N.B. This does work on PC, but I've not tested it on consoles as I only play D3 on PC.