Since both the gain on the shim side and the FFB volume are set to 100, I thought about setting min_force on the custom file side as well to boost it.
Ok, that will not help.
If the in-game gain is at 100%, then it already produces as high a signal as is possible from the game.
Then if you feel it is too weak at the center, you can raise the LFB slider.
Let me know what it is now & how high you try it too.