The difficulty of CK2 is somewhat exaggerated to my opinion. It's just that the UI isn't particularly good, and don't make a great job at telling some mechanics. Watching a tutorial series, or a let's play, is enough to get the hang of it. Reading History isn't necessary. The mechanics don't simulate History, and with the knowledge of the mechanics it's enough to understand what's going on. To be honest, my knowledge on the legal and political system of the HRE, and the Hapsburg didn't help me at all for CK2 or EU4, nor did my knowledge of the Austrian Empire helped my for Vic2, since it isn't simulated.
My favorite game was with House Orsini, count of Orvideo, vassal of the Papacy in 1066 start. I stayed as a Papal vassal during all the game, except when I became King of Jerusalem, which I gave to the Pope, before asking to return to him as a vassal again. I managed to get couple of dozens of my kinsmen as Pope, which is pretty good for a duke. Conquered Egypt, Tunisia, Syria, Sicily, Venizia, and Croatia for the Pope, and other members of my dynasty. And surprisingly, even as a count I was able to marry into Royal family, I suppose being a vassal, and council member of mister Pope, helped a lot. With that I was able to spread my dynasty all over Europe.
If you're interested in good mods, I'd recommend :
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CK2 Plus. It make the gameplay more interesting and challenging.
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Geheimnisnacht, a Warhammer Fantasy Battle, with a magic system. How cool is that? It's super cool. And you even have the Skaven's Under Empire! That's super awesome. And there is Chaos, Vampire, Tombs King, Bretonnian and all sort of great races to play.
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After the End, set in a post-apocalyptic America, where civilization emerged again in a new feudal age, with a bunch of really interesting religion to play as.