Splitting your map to tiles for level streaming is the most common and viable way to do so. I can only speak from high-level but I haven’t seen any game using lightmaps for huge world.
I can say out of experience that Lightmass makes absolutely no sense for large open worlds! We tried it as well and there are just too many limitations. Even if you can bake everything (which will be almost impossible in a huge world with tons of objects) you will run into texture streaming issues at one point.
To put this into some context…we, at our company, havent had any success with lightmass for DI2. And we have quite some resources available
So we went for a dynamic setup (which btw most of the released open world games do as well) and started tweaking things in the engine to better support this.
Check out this thread https://forums.unrealengine.com/development-discussion/rendering/26518-how-does-one-build-lighting-for-massive-worlds