So randomly I’ve stopped getting the 3 rep every time I kill an NPC in the halls of atonement area. Is there some hidden daily cap for how much you can get at a time or something, killing the inquisitors still give rep, yet that takes a very long time to get enough shards for one sinstone.
Do Halls of Attonement. to Honored.
Open World Farming: Neutral to Friendly
While farming mobs in the open world awards less reputation per mob than dungeon mobs, it can be a more lucrative way to farm 
Sinstone Fragments before reaching Revered, as long as the open-world area is not too crowded.
Killing hostile, non-elite mobs awards 3 reputation and 2-4 Sinstone Fragments per kill. Elite mobs award 5 reputation and 4-7 fragments per kill.
Once you hit Friendly, you will stop getting kills from farming open-world mobs.
so then its best to spam halls to honored then?