Who were they - each of them - as individuals?
Personality, family and so on.

Ive always had a love for antiheroes, heroes gone bad and all that.

For example Krosis which is the dragontongue for sorrow, I could Imagine this being a lowborn peasant that had a prowess for magic and became a dragonpriest to protect his family only to become twisted by the magic the dragons bestowed upon him and forced to horrible acts killing hundreds of innocents just to make sure his own family was safe. That is ofcourse just what I wouldve wanted to be offical lore anyway.

Its really annoying that Bethesda dont have a book ingame or official lore regarding who these dragonpriests were as people and why they decided to become dragonpriests.

Am I wrong, and IS there lore regarding them - or didnt Bethesda have the time or "need" to give them more backstory?

Lowborn peasant? Definitely not the vibe I get from a top tier of probably aristocratic priest-kings ruling over humanity.

Didnt want to make a wall of text and take each one of them. But I agree fully with you as to most of them being described like you said, but as "Krosis" meaning sorrow that was more an exception. There isnt even a book describing their past even briefly in the game?

Not that I know of. Sorry.

Their masks enchanted effects may indicate what skills they may of had in life, but beyond that, there is very little on any of them.

Skyrim has better lore, but the MQ (aka important) stuff is still cliffnotes. They made up a language and somehow that it was a good substitute for a story.

It's not that strange that we know nothing. The dragon wars were just rumors before the return of the dragons nobody really knew anything about it, the priests weren't even the most important figures in the war so it's logical to not know anything about them.

My theory:
The names of the dragon priests are not really their names but titles. (Yes, I know of "Otar the mad" but he could be an exception,and his true dragon priest name would be Madness in dragon tongue) bestowed upon them for being champions of Alduin. Let's round up them for better closure:
1. Hevnoraak - meaning Brutal.
2. Krosis - Sorrow
3. Morokei - Glorious
4. Nahkriin - Vengeance
5. Rahgot - Rage
6. Vokun - Shadow
7. Volsung - Air Horror?
8. Otar the Mad

Now. When you collect all the masks you can get the 9th one - Konahriik - Warlord. As I see it. A dragon priest was handpicked by Alduin or other powerfull dragon to be an avatar of their power. His new name would represent the Dragon kind and their dominion. In times of War these few dragon priests of great power would gather in the earliest capital of Keizaal (Skyrim) - Bromjuunar (North King Servant?) to choose a warlord - a Konahriik to lead the forces of men into battle. That makes sense that only when you place all of the masks , you get the final one.