Solved mostly as per solutions I’d considered above. Also removing the Skylight gave me the darkness of the mesh I’d wanted.
Messing around with the warehouse exterior I’m finding pointlights aren’t illuminating the meshes much anyway, which I’d already noticed in other maps. Increasing their LightSource Radius doesn’t do much, and Brightness they blaze. After a lightmass build their light on meshes seems sucked out. PointLight Moveables instead are working well, illuminating as I’d expect lights to illuminate. It’s not always the case though, sometimes they’re fine. In future I’ll be trying the basic pointlights first. If they don’t work - when I want a contrast of light on meshes to darkness beyond - I have the option to use moveables. And I’ll experiment further.