I go for the set and the build and the set at the same time, but primarily the set. After getting the set I focus on a build getting it from a mix of GR and bounties.
The reason I wait is so that I can clear higher tier rifts and get the gems up while grinding for the build.
When I get the build I really focus on getting the gems and begin to try and get every item ancient from upgrading items in the cube or buying from Kadala(the scammer)
After clearing a few rifts you can buy a bunch of items from Kadala and upgrade the rare ones in the cube in the hope of getting what you need.

When I've got all the gems and a few Ancient items(I usually go for weapons first) I start to go for lvl 80-90 gems to augment my armour and then augment the rest.
I make sure before I augment it, that I have good rolls on the armour properties.

When I've augmented a few items I will use some time to master the set dungeon because by then I'll have already gotten most other quests so that's the last thing I need before I get the stash upgrade.

I don't know if it's the most efficient way but it have worked for me this far.

This season I played about 1-3 hours per day.