I might be able to speak to some of that, I follow UE5_main relatively frequently and have been building experimental versions to test some new features.
To address improving lumen quality, the easy levers are of course to increase the final gather and reflections quality in the post-process volume. This generally improves the resolve of GI and reflections especially. The rock is black in your scene because geometry below a certain height and distance threshold are culled from the lumen scene for performance reasons. You can remedy this by increasing the lumen scene detail value. Furthermore, I note your scalability is set to high, which would explain some of the lower quality. High targets open worlds at 60FPS, which means near-field detail and reflection quality are degraded. Increase your settings to epic and you should notice an improvement. It also doesn’t look like non-SS GI is showing up in your lumen scene entirely, what version are you using?
There is in fact translucency, but it’s rather meagre in 5.0. Low-resolution radiance probes are used to shade translucency (in addition to screen-space reflections), and while that avoids light leaking, it means the translucent reflections are noticably dull and low-res. In 5.1, which is in production, Lumen supports high-quality translucent reflections, which are indistinguishable from the regular reflection modes. I’ve tried them out myself and they are effective and performant.
As for improvements, there are quite a few coming down the pipe for 5.1. Solutions for over-occlusion on foliage and other alpha-tested geometry, support for subsurface scattering and two-sided foliage shading affecting GI, improved reflections definition, scalability and performance, light-leaking solutions, and more, in addition to massive performance improvements in general.
I’ve tried out most or all of these features, and I’ve found they all work great. When 5.1 will actually arrive is something I don’t know, so it means these features remain walled-off to the people who know how to build UE5 and are comfortable taking risks on an unstable engine.