Reinstalling it from scratch huh ! I admire your dedication @Nebulon_Ranger 
Alrighty, that means two things then :
1 - There is one very specific setting (out of the few confusing ones…) that allows to plug either that one pre-compiled pack or a user-compiled one, for sure. I’ve tried a few different things myself and overall it is indeed working (as I haven’t seen any lengthy compile in days), but I am not sure which one of my settings did the trick specifically.
2 - There seems to be something else going on with the per-project Saved and DerivedDataCache folders too, as I am positive that copying these over from one project to another solved the second round of shader compilation for me (the one that is shown on the bottom right corner of the editor, post launch). But I may be remembering incorrectly.
FWIW here are my settings. I’ve plugged in four different paths (one for “Global” and “Global Shared”, and one for “Project Local” and “Project Shared”) and I honestly have no idea which one did the trick, nor do I know if it is pulling from the pack shipping with the engine or my own compile of it. But the little Derived Data icon is green, so I suppose there is at least one correct setting out of the four
(Overall I am still completely amazed that such a high-end tool doesn’t come with a reliable configuration wizard to make sure that these things are setup properly. I understand that the user is supposed to be technically minded, but here in this case I am pretty sure that hundreds of thousands man-hours have been wasted by this nonsensical shader compilation issue. And with online forums being pretty much dead now, it just remains unsolved and mostly not reported. This is incredibly frustrating !)