No bash or exiftool needed, just use the daylight mode in the white balance module ?
If you have the ‘modern’ color workflow enabled , you could disable it, or do what I said and disable 'color calibration ’ after it. To get a quick fix.
You are right in that the white balance mode is just a bit of metadata, so the image your camera takes is actually exactly the same bo matter what you had set as the White balance mode . It just writes a tiny bit of information what the white balance mode was, and darktable (and other raw processing software ) reads that and uses it to present a default .
Bit that’s a ‘default’ , nothing more nothing less. You are free to change it, with exactly the same results as you would get if you start messing with exiftool. But I wouldn’t touch your original files , And it’s easier to set the White balance in a tool like Darktable then it is to do from a command line (talking even as someone who is very comfortable on the command line ).
So, open one image. Turn off 'color calibration ’ if it’s enabled. Set white balance to daylight (or auto , or something else you prefer). Create a style (preset) with the white balance module settings in it , and the 'turn off color calibration ’ in it . Now from the light table view, select all the images and apply the style . Every image is now at daylight with no further edits made to it .
If you really want to use the color calibration module , you’d have to mess in there to set a 'normal daylight '. I actually don’t know from the top of my head how , since I don’t use it anymore 
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