Hi Thomas, welcome to the forum.
I wrote a general introduction to color management when processing raw files that might be of some help:
https://discuss.pixls.us/t/article-color-management-in-raw-processing/11521
One thing to clarify in your original post, the ‘standard matrix’ for your camera in darktable and the ICC file you created from the target shot should be quite close in performance, as both contain a 3x3 matrix of the camera’s color response, both (in the case of the standard matrix, “probably”) created from similar 24-patch ColorChecker targets. Your profile created wtih scanin and colprof contains just another 3x3 matrix that is used instead of the internal one.
Now, the deltaE (I’m assuming it’s the worst dE of the 24 patches) you got for your three cameras is actually quite good for a 24-patch matrix profile; I typically don’t get better than 3 dE from target shots. I use dcamprof to make camera profiles, it’s a command-line program written by one of the RawTherapee developers, and among other things it spews out tiff files showing patch dE comparisons in a real photo, and 3 dE is actually not that easy to notice.
I’ve done a lot with camera profiling, including measuring my cameras’ spectral sensitivity (another profiling rabbit hole), so between me and the others here who’ve done similar work we can answer your questions confidently.