It seems the Fenix 5x plus firmware may not be in traditional Garmin format therefore it's not recognized by GarminCure3.exe for patching of file names. Check if it's encrypted because if so there's no help for that as far as i'm aware.

Have you tried using QuickCure3.rgn? If not and you want to try it, [Only registered and activated users can see links. ]. Rename it to 290001000154.rgn and flash in preboot (at your sole risk of course, i don't know the outcome).

Another way if it's not encrypted firmware is to try the old original method we used before kunix made his cure3 app. Done by using a hex editor to rename IMG to MIG for example in the firmware RGN to make a cure firmware specifically for by-passing loading map image files on boot. See post #4 here: [Only registered and activated users can see links. ] Also this thread: [Only registered and activated users can see links. ].