If I have all the pieces I need, but 3 or 4 aren't ancient, will I do enough damage to switch to LON, or should I just grind mats using helltooth set and reroll all the non ancient pieces before I switch?
No other gear can be a set other than the two rings.
For every piece that's Ancient you will get a boost.
Now - since each piece is Unique, you want to have pieces that have abilities build into them - i.e. Nemesis Bracers - activating a pylon will summon an Elite pack. You just need to put together pieces that work well together. So using random gear is not going to cut it.