While farming covenant items for achievements (PvE) it became clear that the Vertebra Shackles, Proof of Concord, and Sunlight Medals have dramatically lower drop rates - farming each individually was taking damn near forever.
What seemed to help! - cycling between the three respective farming zones and switching after a covenant item drops (also don't know if it's just me? but equipping the respective covenant insignia seemed to help)
Sunlight Medals - Lothric Castle bonfire killing the three knights headed back towards the dancer
Proof of Concord - Anor Londo, first two knights up the stairs very easy to evade and backstab third knight is a bit sturdier so personal judgement call
Vertebra Shackles - Catacombs of Carthus, turn right from bonfire and kill two nearby shield swordsmen and one curved greatsword at end of hallway. Rinse and repeat or jump down and take out other swordsmen on the way to Wolnir. Two handed swordsman overlooking the broken bridge seemed to drop often for me.
(Soul level 229)
At 40 Luck (50 with Hollow Longsword +10 and Hollow Wargod Shield +10 equipped) and Symbol of Avarice / Covetous Gold Serpent Ring +2 along with plenty of Rusted Coins, didn't take me more than a day or two to get all 30 items for each covenant.
Had to share - hope it helps someone.
Praise the sun *****es