Regarding the firefly and continuity of professional renders. Are you talking about videos or stills? I rendered in 5.0 for my last portfolio project and at least the stills did not seem to vary (with lumen).
Moving videos. I think mainly the issue is with Lumen+Virtual shadow maps.
Yes, I noticed that too and it is really annoying. Lumen also can be tricky for indoor scenes with emissive light sources. Too many artifacts -> immersive feeling gone.
Yeah, I wouldn't use lumen for interior spaces unless they are almost completely lit from the sun/incoming light
5.1 solved some problems with noise, interiors, and GI in general. Also, reflective and translucent materials look right now. Nanite works with thin meshes like tree leaves and even improves performance. All improvements are in "Unreal Sensei" channel video about UE5.1 changes.