It's cheaper to re-roll rares than legendaries, so roll the enchants before applying a legendary affix
Helltides are the only drop locations for the mats you need to re-roll enchants
--Helltides.com has got a tracker for when they'll be and where the Mystery chest is.
You can do the capstones at any level, but if you're early it could be tough.
The nerfs hurt, but still fun to play