Yeah making it even mostly hereditary is a bad idea because nature doesn't hand out guarantees like that, and exceptional natural talent in particular is too fragile a balance of factors, it falls into mediocrity very easily.

The longer periods of stability seemed to come when the aristocracy had some churn, families fell from grace and titles were stripped, expansion and growth left room for new people to be promoted up, and frequent conflict tested the mettle of many and let new blood distinguish themselves and be chosen by achievement.

The greatest source of disorder I think is from the natural desire for the best for your children, so those in power do what they can to make sure their children get to be in power too, even when they're not fit for it. We could do with a mythology that idealises being a noble leader based on early life achievements, but also a society that doesn't allow those leaders to live a more luxurious lifestyle than the rest. So that when a parent recognizes their child isn't fit to follow in their footsteps there's not a conflict of interest in not having their children also lead.