Thank you for referencing my post! I’m happy you found it potentially useful.
Everything you said was spot-on, RT reflections with lightmass is currently the only way to achieve good-looking mirror reflections. Both lumen GI and reflections are high-quality for the first bounce, and then reduce fidelity significantly. It works great for most game use-cases, but breaks in many Archvis scenes.
And the path-tracer is an interesting beast, it supports full dynamic GI/absorbsion/transmittance, and can get pretty much the best results you need. Lumen can match the path-tracer’s diffuse lighting identically, but not anythiing specular.
Good news down the line though: lumen reflections for Lightmass, and multi-bounce specular are on the list of features they want to add, it just may take a while.