Inspired by /u/briansey's post, I decided to do something similar. Since my PC crashed a day before the season an I couldn't fix it in time (plus preferred to watch the World Cup), I missed the season start. So I saved up 3 Challenge Rift caches (which means I didn't have to run any bounties), got a power level to 70, and wanted to push my luck. As I managed to find a spot in a T13 group as "rift closer", I managed to complete the UE set at around 2-3 hours played. The rest was pretty simple: I used a certain Journey Tracker website to keep track of my season journey process while leaving Kulle out, until the end:
All 3 conquests (3 gems to 65, TX rift in under 2 minutes, GR75 solo) were just pickups along the way. In the end, it took me 8:15. After I got UE everything was solo; but I spent about an hour or so chatting with one of the power levelers in-game, so this is by far not the fastest possible (of course if you get carried the entire way this is probably doable in 2-3 hours or so). Learned some interesting things though - for example, make sure you have 200+ DB and some random legendaries to cube the final 40 items; leave the one bounty until the end, so you can join a random T13 bounty group; when being the rift closer, make sure you maximize toughness+speed so you can pick up DBs+items in the rift; and probably more that I forgot now (not sure if anyone's interested).
Doing this entirely solo would've probably taken about 15 hours: 3-4 hours for getting to 70 (I'm not good at leveling solo, especially DH), then 2-3 hours getting the full 6 piece, and then an hour more because I would've missed out on the ~10-15 rift keys I got as T13 closer.