[Questions] How to choose a DCP profile (if at all)

Really? If you want, here are mine:

Measured RGGB data + test overview
$ exiftool -'Measured*RGGB*' ./*.CR3 | awk -F ': *' '{ print $2 }' | column -t

# “Normal” picture, not taken just for tests:

1850  1024  1024  185
641   1024  1024  586
744   1024  1024  1087
499   1024  1024  612
629   1024  1024  514
333   1024  1024  871
620   1024  1024  612
559   1024  1024  556
710   1024  1024  351
609   1024  1024  362
556   1024  1024  486
605   1024  1024  564

# Test series:

623  1024  1024  590 # CR3, AWB
618  1024  1024  592 # CR3, Fluorescent white
622  1024  1024  592 # CR3, Custom (assuming I did the calibration correctly…)
620  1024  1024  592 # CR3, Forced 4000K
648  1024  1024  580 # RAW, AWB
626  1024  1024  589 # RAW, Fluorescent white
630  1024  1024  588 # RAW, Custom (assuming I did the calibration correctly…)
637  1024  1024  587 # RAW, Forced 4000K

Here’s an overview of the embedded JPGs of the test series:
penguin-thumbs-merged

Perhaps RT relies only on that and there’s another field somewhere that needs to be used to tweak that? These values are indeed always often lower than 1024. At least with my camera, but also with two raws I got before my purchase to try stuff. One from raw.pixls.us and the other from that other topic on this site.