I understand you because I had a hard time trying to figure out the best way to recover highlights with filmic rgc.
After watching some videos of Aurélien and reading/learning about the development of the filmic rgb module in this forum I’m now have a way to recover highlights and I’m very satisfied with it.
Basically I deactivate highlight reconstruction altogether, and set preserve chrominance in filmic to ‘no’. After this I go to the reconstruct tab of filmic and adjust the threshold and maybe some of the other sliders. Part of those steps were an advice of @kofa in this thread: Lost highlights in darktable - #2 by kofa