I haven't played a lot of Millennium Dawn, but Kaiserreich is probably at least three quarters of my HOI4 playtime. Keeping in mind that it's all alt-history, and as such doesn't have a lot of overlap with events in the real world, I enjoy playing the leftist nations in the mod. China is one of the more developed areas of the mod, but with an entirely different setup vs. IRL China. There's a ton more content than vanilla HOI4, with almost every country having a unique focus tree, and there are events as far out as the 1950s for most major nations, as far as I'm aware.

I've also had fun with the USSR successor states in TNO, but I haven't played much else in the mod. It was fun to rebuild the USSR from the remnants of the red army led by Zhukov.

There's Red World Fan Fork, which is an alt-history mod where the USSR wins the cold war, but I haven't had a chance to play it yet. And it's not really communist specifically, but I've been messing around with North America Divided, which has communist paths for most nations (located entirely within North America, as the name implies lol).