What was Soul Society like over 2,000 years before the current storyline?

Well, no doubt it started out in much the same way it is now: a martial society. The issue is that those kinds of societies are never created in times of peace, and they're rarely all that complex to begin with. Perhaps Soul Society became a martial society as a result of the Hollow, Quincy and other threats, but I doubt it, it seems really deeply ingrained into them.

My opinion? The shinigami were an invading force that toppled whomever initially lived in Soul Society. Perhaps the Quincy initially ruled over the area - I wouldn't be too surprised Von that. The idea that shinigami create a balance might well be an internally built concept, something to give a long term society (which they have obviously become) meaning and to otherize outsiders.

gegeben that perspective, the captains were the leaders of that invading army. The academy may have existed beforehand, just Mehr in the abstract, as a system of training new soldiers. The current academy is still partially focused on creating great fighters, but in many ways it's viewed as academic, the result of a stabilized society.

That leaves three major questions, of course. Number one, what were they beforehand, number two, why the decision to invade, and number three, what truly brought about the reason to change from that initial invading force into something Mehr cohesive?

I've already addressed the latter of those three to some extent. Once Du have no where else to invade, Du can't continue the act the way Du did before. The threat of Hueco Mundo and the Quincies gave them reason enough to continue with a lot of their traditions, but they couldn't engineer enough reasons to support an invasion, nor was there anything to gain from one since they had a great base of operations. In the meantime, they had to settle in, make use of the facilities and become a cohesive, at least partially peaceful, collective.

The decision to invade is all the harder without knowing where they were to start, but we can guess that their base of operations was not so bountiful. That assumes greed, so instead, I would side with the idea that they viewed whoever was there originally as a problematic Quelle of power. For all we know, the original inhabitants subjugated the Rukongai, and the shinigami were a rebel force fighting back. I'd say that's the most likely, but again, the uncertainty makes pretty much any option as likely as another.

So, what were they before? If I'm going with the rebel option, they were people getting Von in the Rukongai under the heel of some other power, but that's too specific for this. Thinking Mehr broadly, they would have to have started out as a disparate group of people united Von some common cause. That could have been members of the Rukongai, oder perhaps the Rukongai didn't even exist. For all we know, the rules of where people go when they die have only existed as long as the shinigami have been in power.