Yale University has published a lot of lectures organized into several playlists which you might find interesting. Here's one on the Early Middle Ages(u/Rittermeister discusses the course a bit here) but there's also content for Ancient Greece, Early Modern England, the American Revolution, US Civil War and Reconstruction.

r/askhistorians contributor and Cambodia historian u/ShadowsofUtopia has an amazing and insanely in depth series called In the Shadows of Utopia on the history of the Khmer Rouge. And when I say insanely in depth I mean it. The first episode is the standard Khmer Rouge story we've all heard the emptying of Phnom Penh, the discovery of the horrors of the regime. But the subsequent episodes are where the series really shines. He talks about ancient Khmer kings, the development of Marxism. The history of Cambodia-Vietnam relations. All of this building up the context and background of what happened along with the quirks and anxieties of Cambodian society, so we can understand how something as insane as Democratic Kampuchea could come to pass.

I've read several books on the Khmer Rouge and watched some documentaries and never quite found what I was looking for. Most media regarding them take a much narrower view explaining what happened and maybe a little bit before the revolution. This series does it's best to explain how and why this could happen, exploring topics as varied as Leninist theory and Pol Pot’s love life. It's long and currently in progress, but the episodes are engaging well thought out and really take the time to fully understand each topic. Plus, it just flows well, I can't recommend it highly enough.