Have you checked out the Lumen Team’s Siggraph presentation? Wright shared a link with me in the forums, and they also had a presentation on how ray-tracing large scenes works.
From what I’ve read and understood via the presentation and Github comments, they have already implemented perf. improvements for hardware lumen that didn’t make it into a few of the UE5 demos (Fortnite I don’t know about). It includes how they handle distant geometry via the far-field, how BVH trees are constructed and streamed, and a few other things. They did mention that that performance on PC may be improved when Unreal Engine gets bindless resources for ray-tracing, so there is room to go.
I agree with you that 4A’s implementation of RTGI in metro exodus is extremely competent. From what I’ve learned/played however, their system only robustly handles diffuse, opaque geometry; shinier, transparent, and alpha-masked geometry see limited or no support. Lumen trades performance for feature robustness in this case, but there’s nothing to say the tech couldn’t be made more scalable.
In terms of mixing SW/HW RT, I wouldn’t know the performance implications necessarily, but I’m curious what you see as the benefit. Lumen can hand off between different ray-tracing methods, so I suppose the performance would determine how useful it’d be.