I'm sorry to say this but... that's insane!

The only useful way to get the pieces for the two quests are chests.

I hunted 168 Dwaven Ghosts and I got 4 Dwarven Relic pieces. And not even rare ones but the more common.

My calculations might be wrong but for me there are the following drop chances for the whole things (tried to calculate it for each set):
Allathos' Completed Journal: „Passage of Time“
Allathos' Journal 8th Awakening, 2157 B.R 46,51%
Allathos' Journal 12th Awakening, 2157 B.R 27,91%
Allathos' Journal 15th Awakening, 2157 B.R 10,47%
Allathos' Journal 17th Awakening, 2157 B.R 12,79%
Allathos' Journal 18th Awakening, 2157 B.R 2,33%

Allathos' Completed Journal: „Travels with Nelthorr“
Allathos' Journal 52th Awakening, 2157 B.R 40,22%
Allathos' Journal 59th Awakening, 2157 B.R 25%
Allathos' Journal 74th Awakening, 2157 B.R 17,39%
Allathos' Journal 114th Awakening, 2157 B.R 11,96%
Allathos' Journal 132th Awakening, 2157 B.R 5,43%

Nellthorr's Completed Journal: „The later Years“
Nellthorr's Journal 115th Awakening, 2157 B.R 28,28%
Nellthorr's Journal 121st Awakening, 2157 B.R 34,34%
Nellthorr's Journal 125th Awakening, 2157 B.R 23,23%
Nellthorr's Journal 131st Awakening, 2157 B.R 8,08%
Nellthorr's Journal 150th Awakening, 2157 B.R 6,06%

Nellthorr's Completed Journal: „Travels to the East“
Nellthorr's Journal 32th Awakening, 2157 B.R 34,02%
Nellthorr's Journal 33rd Awakening, 2157 B.R 28,87%
Nellthorr's Journal 45th Awakening, 2157 B.R 14,43%
Nellthorr's Journal 51th Awakening, 2157 B.R 11,34%
Nellthorr's Journal 64th Awakening, 2157 B.R 11,34%
(which means if I'm lucky and only get things for this set, I'd need a maximum of 2205 pages to craft 250 dragon journal sets and lets say I get one useful page out of one chest, there are max. 8 chests in the caves, means 8 pages per run, means 276 cave runs and much money for Knossos selling the keys.)

Dwarven Relic Pieces (obviously not for each set ;)
Amulet of the Tribunal 12,70%
Ancient Figurine of Grathmar 1,06%
Ancient Palmyran Scroll 6,88%
Dwarven Axe 4,23%
Dwarven Dagger 12,17%
Dwarven Helm 4,76%
Dwarven Marriage Band 7,41%
Dwarven Scabbard 4,23%
Dwarven Shield 12,17%
Figurine of Brobbet 10,58%
Golden Coin of Aughundell 0,53%
Sapphire-Studded Earring 8,47%
Scarf of Palmyran Steelsilk 6,88%
Shard of Satyr Pottery 5,29%
Token of Trandalaran Might 2,65%

As you can see, some items are so incredible rare that it'll take aaaages to complete the 250 sets. Is this really how it should be? I mean... let's look at something else that's not for a quest (or at least not for an emblem quest):
Sigrun's Letter, Page 1 6,79%
Sigrun's Letter, Page 2 8,60%
Sigrun's Letter, Page 3 10,86%
Sigrun's Letter, Page 4 11,09%
Sigrun's Letter, Page 5 7,47%
Sigrun's Letter, Page 6 8,82%
Sigrun's Letter, Page 7 6,79%
Sigrun's Letter, Page 8 5,88%
Sigrun's Letter, Page 9 9,95%
Sigrun's Letter, Page 10 7,69%
Sigrun's Letter, Page 11 9,05%
Sigrun's Letter, Page 12 7,01%

which is an average of 8,33%. Lots of numbers now but... I hope someone will agree with me, that this is like a neverending task if one doesn't want to roam all the chests in the caves for endless weaks. It took me a long time to get even the first of both emblems for 50 and 100 sets.

Again: my calculations also might be wrong. I just collected many things (for example 189 dwarven relic pieces were in my inventory, or 374 dragon journal pages (over all 4 sets)) and then just looked which one dropped the lowest from each kind.

Now it's time to rest and dream of dwarven relic sets that attack me *smirks*