If you want to keep it optimized, you should split it up into rooms, so occlusion culling can do its job. But on the other hand, we are talking about like 1000 tris that barely puts any stress on your videocard (or any device) so I'm not sure if it worth the effort.
If you would want to reuse the pieces and build something bigger, a modular approach is always the best. But if you would only want to use this as it is, I'd say its completely fine as one mesh.
About the lightmap resolution thing: Unreal will re-pack and group things when it comes to lightmapping. It will create an atlas of lightmaps. You can set the atlas resolution in the world settings\lightmass. So you'll get one huge lightmap anyways.
If its not going to be reused and you don't want to build multiple differently shaped rooms, so its not a modular thing, and you want to split it, I'd split it by rooms. So a room would be one mesh including its walls,floor and ceiling. There is not much point of splitting it into that small pieces as walls, floors etc, unless you want modular meshes - Now that would make it worth to split. That amount of polygons is literally free. It won't improve anything if you cull the walls and floor and ceilings separately.
As I wrote above, since you have really simple, low poly geometry, you could even leave it as one and it would be fine. But the question is still, how would you want to use them. If you want reuseability, its good to split. If you are not going for that, and you have this simple assets, its fine as one. An another case that would make it worth to split, if you would have much higher poly geometry, like a modeled brick wall or something that is ten-thousands of tris. Then splitting would help a lot because you could cull the heavy assets very efficiently. But your ones are very light.
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If you would want to reuse the pieces and build something bigger, a modular approach is always the best. But if you would only want to use this as it is, I'd say its completely fine as one mesh.
About the lightmap resolution thing:
Unreal will re-pack and group things when it comes to lightmapping. It will create an atlas of lightmaps. You can set the atlas resolution in the world settings\lightmass. So you'll get one huge lightmap anyways.
As I wrote above, since you have really simple, low poly geometry, you could even leave it as one and it would be fine. But the question is still, how would you want to use them. If you want reuseability, its good to split. If you are not going for that, and you have this simple assets, its fine as one. An another case that would make it worth to split, if you would have much higher poly geometry, like a modeled brick wall or something that is ten-thousands of tris. Then splitting would help a lot because you could cull the heavy assets very efficiently. But your ones are very light.