My editing improved when I came to appreciate this. But it only applies to the raw file, once the image has been converted to a tiff like the OP does then the white and blacks are locked in.
With AgX, I find I usually push the shoulder power all the way to the right (value 5) to get enough contrast in the highlights. This is easier to get contrast in the highlights than sigmoid or filmic RGB.
shoulder power is very forgiving for this. Toe power is a more delicate operation determining how dark the dark looks. Certainly worth the OP’s effort to experiment with if he tries AgX.