For DLC, I just bought 1-2 at a time and treated it like a new game each time. I now have most of them, and I've always waited at least a year before buying a DLC to get it for 50% off.
I think the UI is fine, though it seems EUV is coming out soon, so the UI will likely be completely reworked. If you're thinking about jumping in, I recommend waiting for EUV since EUIV will likely be steeply discounted (just like CK2 was when CK3 was released).