Shadow softness will be based on shadow method and the lights source radius (not attenuation radius). It doesn’t really have anything to do with Lumen, as direct shadows are handled by the aforementioned shadow method which is set completely independent of lumen.
Lumen does control indirect shadow, however - and indirect shadow tend to be very soft. This is because you can sort of think of all objects that bounce light as large light sources with respect to the bounced ray. A wall is kind of like a giant rect light, as light from even a single point will hit across it’s whole surface and then bounce across a large area.
With that said, backrooms games are probably a perfect example of a game style that is likely better off using primarily baked lighting. I’d question the value of real time GI at all if your project is full of mainly just static lights and geometry. But I’ll leave that to you, I don’t know your projects needs.