Blender 2.8 boolean modifier messes up lightmaps

Hi everyone!

I’m very new to UE4 and blender, so please bear with me.
I’m trying to create a Bauhaus style building in blender 2.8 to add in my UE4 environment, with a garage gate that has trapezoid inlays (see attached pictures). In blender, everything looks OK, but when I import it in UE4, everything is messed up (also, see pictures).
What really drives me nuts, though, are the lightmaps. If I apply the modifiers before exporting, I get the shown results and the messed up geometry.
If I don’t apply the modifiers, I can’t adjust the lightmaps.

How do you guys deal with messed up geometry caused by boolean modifiers in blender (or how do you avoid the mess)? And mostly, how do you adjust the lightmaps? The lighing is currently my biggest problem (I think) and I haven’t found any good, easy to understand tips in the forum, answer hub or youtube. What I’ve found is either too specific or too simple.
Can someone maybe give me a link to an intermediate lightmap tutorial for dummies? As I said, I’m new to the whole 3D shebang and already way over my head by just trying to import something. x_x
Any help is much appreciated!