Well...? How are the linear previews in display space?
For reference, Mark's colour checker image was processed in darktable with only basic processing options applied; i.e., black point, white balance, demosaic, input profile (standard colour matrix), linear Rec2020 working profile and display encoding (sRGB).
The raw colour checker file was converted to fits in Siril, bias subtracted, debayered, white balanced and a sRGB CCM applied before saving to a 32-bit fits file. This represents a linear-gamma sRGB image as it would be processed in Siril.
The linear-gamma sRGB image in Linear preview mode. As expected, the image is rendered too dark.
The linear-gamma sRGB image in Linear preview mode with the sRGBlinear ICC profile assigned via the command line. The Linear preview now matches the reference.
Finally, the sRGB TRC was applied to the linear-gamma sRGB image using Pixel Math and saved as 32-bit fits. This represents an sRGB image.
The sRGB image in Linear preview mode with the sRGB ICC profile assigned via the command line. The Linear preview also matches the reference.