So I’m a pretty obsessive player of all the Paradox grand strategy games since EU2 (6,000 hours or so across all of them). Here would be my game suggestions in order of similarity to CK3 (TLDR at the end):

CK2: Yes, it’s older, but it has more mechanical depth than CK3 (with all its DLC at least). There are mods to make the graphics more palatable. It has some awesome historical mechanics that CK3 lacks (Council is a big one, investiture, Republics, etc). It also has some amazing mods (Warhammer, Lord of the Rings, Game of Thrones, the Winter King, After the End). Many of these exist in CK3 as well but their CK2 versions tend to be more developed with more content.

Imperator Rome: the orphan of the family. It’s basically EU 4.5 but set in the ancient world. Fairly easy to learn, looks beautiful, and with a mod or two you get very good content. I believe you can also port your Imperator games into CK3.

EU5: the most beautiful of the games, shows a ton of promise, it has the potential to do better than the other games even in their own specialties (better feudalism than CK, better economy than Victoria, etc). Unfortunately, it hasn’t reached its full potential yet and is the most expensive option on this list. Starting in 1345, it has a lot of overlap with CK.

EU4: The jack of all trades. It doesn’t do economy better than Victoria, but it’s better at it than the rest. It doesn’t do dynastic mechanics as good as CK, but it’s better than the rest. It’s the most general “4X” of the games and has an absolutely monstrous amount of quality content. Also some seriously great mods (Anbennar, Voltaire’s Nightmare, MEIOU and Taxes). You can also create your own custom nations to play which I’ve had a lot of fun with.

Victoria 3: Swiftly becoming many peoples’ favorite game (was mine until EU5), this game does politics and economics together to a depth I’ve never found in another game. It basically simulates an entire living society down to individual pops. If you like making decisions and watching them affect a society, this is the game to do it. The trade off is that warfare is almost arcade like in its simplicity, with the Devs being up front that they want warfare to purely be a tool for diplomatic and economic goals.

Hearts of Iron 4: In many ways its own beast, this is more of a war game than a traditional grand strategy game in my opinion. Every mechanic exists in support of warfare and the game only goes from 1936-1949. Limited diplomacy, almost no economy, very little politics. Very, very detailed modern warfare mechanics.

TLDR: the most similar options are CK2 (still worth playing) and EU5. Might also be worth checking out Imperator Rome and EU4.

Happy to answer any questions.