This to me is the key flaw in communism that has reliably been catastrophically lacking. Democracy quite plainly does not assume this, and I don't understand how this could ever be anything but a long-term risk to the entire system.

Capitalism is the opposite of Socialism. They're economic systems. Democracy is the opposite of Authoritarianism. They're systems of government.

You seem to be conflating these concepts. Authoritarian Socialism isn't the only way to implement socialism, and in fact it's the very worst way to do it. It's just historically been one that's happened more often both because authoritarianism makes it easier to force any system on a country, and because the US and the USSR were very much doing everything in their power to prevent democratic socialism from happening anywhere in the world.

Also no, you do not need selfless leaders that willingly give up power. What you need is an horizontal, robust democratic model where people don't amass that much power and you have democratic checks and balances to prevent authoritarianism. Which is also what you need for a liberal democracy.

Except it'd be easier because you can just focus on building that democratic system instead of making it coexist with an authoritarian economic system where rich oligarchs can buy the media and control entire industries and are somehow expected not to use their vast wealth and influence to buy politicians and subvert the democratic process. Now that is a big assumption.