You could just make your assets that are never going to move stationary (not the light setting, the actor setting, like your Trees or whatever). Then the light would be set to Movable (dynamic). This is used in Fortnite as a way of optimizing dynamically lit scenes that have a lot of geometry that’s not really moving.
Read up on it and see if it’s a fit for your needs. I only read your initial post and wanted to share this feature you’re likely not aware of. Sorry if I missed something else you had posted in here.
Some other things you can look at too.
For the far shadows, you could enable Mesh Distance Fields and use RayTraced Soft Shadows: https://docs.unrealengine.com/latest/INT/Engine/Rendering/LightingAndShadows/RayTracedDistanceFieldShadowing/index.html
This will give you the far shadows you want without having to bake the scene. Larger scene typically don’t work as well for baked lighting because of all the lightmaps that have be stored and packed into atlases and then you’re using your texture memory to load these all in when needed. This is why more open scenes use dynamic lighting instead of baked.
So if you opt to go with DF shadows, I would suggest using a mid-low-range value for the Dynamic Cascaded Shadow Maps distance (something in the 1500-2500) range so that you get nice sharp shadows, and when you enable Distance Field Shadows you get the mesh distance field representation at these farther distances.
Anyways, I hope that helps.