You are in luck, because there is a widely available popular press book by a distinguished historian on precisely this topic: The Great Leveller by Walter Schiedel. I don not want to be too long winded because if we are being honest the answer here is "read the book" (or listen, I thought the audiobook was excellent), but in brief he identifies four historical forces that reliably and consistently create real social levelling: mass mobilization warfare (examples range from Warring States China to the Second World War), revolution (of the French rather than American variety, real social disruption), systems collapse (such as the end of the Roman Empire) and mass mortality, specifically from plague (naturally the Black Death takes center stage). The main connecting thread is that there needs to be a real shock to the system, such that elites are either forced to share material wealth (such as with mass mobilization warfare) or their ability to control power and resources is weakened or shattered altogether (such as systems collapse). Unfortunately the counter example he brought up (mass democratic reformist movements in the guise of the "Pink Tide" in Latin America) is outside the twenty year rule, but in short he thinks it and similar movements are not quite able to produce transformative change.

I am happy to clarify any points or respond to any examples, I will briefly mention one thing in your post:

when some new resource or country is opened for wealth generation and established organizations/families are too slow to take advantage of it

Leaving aside whether the "nimble entrepreneur who outplays the old stuck in its ways establishment" is more myth than reality, to Schiedel's analysis this is irrelevant: all that is produced here is a new rich person rather than real systemic change.