I think I have found the real solution via How to make a material with opacity AND other options? - Development / Rendering - Epic Developer Community Forums
Summary, set Blend Mode to Translucent. Then set Translucency->Lighting Mode to “Surface TranslucencyVolume” or “Surface ForwardShading”.
Now opacity parameter can be adjusted while lighting effects are not lost.
I’ve searched a long-time for this solution, and kept hitting this post as the near #1 search-hit but it wasn’t a solution that really uses opacity as expected, not tricks with masked.
Shadows can also be rendered from this with the appropriate material settings enabled such as masked shadows.
Unfortunately, if this is a complex object where it has to deal with depth-sorting elements of itself, such as a skeletal mesh, it may have very visible issues where elements in the rear will draw on top of elements in front – at low-opacity levels such as 0.9.