SUMMARY
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I found settings on how to better integrate Digikam and Darktable, reading and writing each others sidecar files.
It would be nice if those could be native to Digikam, in some sort of profile or a checkbox.
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To better integrate Digikam and Darktable, 3 settings must be done and it takes a *lot* of time and searching to find these.  Most users will never find out that this is even possible.
I'll list these with the notice that this is applicable to working with these two programs. It is not a universal 'make everything better' setting.
I would prefer if there was some setting or button that would make all these changes at once  (and for good measure, perhaps a checklist with which other programs you want to integrate Digikam :) ) 


1. digikam should prioritize XMP files over its database
Ncessary because Darktable can update the XMP files, so the XMP must be leading, always.

2. digikam must read and write tags in the sections and format where Darktable has them.
Here is a very good explanation:
https://userbase.kde.org/Digikam/Tutorials/Setup_of_digiKam_for_Windows_compatibility#Step_2_-_Fix_which_metadata_is_written_into_files

However, it is still missing the "xmp.lr.hierarchicalSubject' - it should be added. 

3. digikam must no longer read/write tags in other sections of the XMP 
Otherwise, when Darktable updates something, Digikam will happily read the old data back from another section of the XMP file.
Example: There is a tag 'elephant' which exists in "Mediapro:catalogsets".
The user removes this tag in Darktable.
When you start Digikam, it will still read the tag because it finds it in a section of the XMP that Darktable did not update.
Found this out the hard way and needed to fix all my XMP files with awk.