I think Stellaris, despite not being a historical themed game, is a better frame of reference for what we could expect for a relatively successful model of one major DLC and smaller minor DLCs (yes there were some releases with issues like Megacorp, but I'd argue these were less due to the model and more due to the quality of the DLC or free patch itself). There are big expansions like Utopia, Apocalypse, etc. that add in new mechanics and a huge amount of new events and flavor in both the DLC and the accompanying free patch that substantially changes how the game plays. Then, there are smaller flavor DLCs - called story packs - like Leviathans, Synthetic Dawn, etc. that add mechanics and events that add a more limited scope of flavor to the game. Additionally, in Stellaris, the free patches are often substantial reworkings of the game that would have been included in a DLC elsewhere, so I don't expect to see the EUIV issue where it's a frankensteinian mess of features tacked onto random DLC that make the game very different if you have the DLC vs if you don't.
So, ideally, CK3 will follow a similar model as Stellaris.