I’d use the sidecar files for that: have Digikam write .xmp files, then copy both the raw and the .xmp from the laptop to your main computer (as long as you put both in the same directory, this works even when the two computers have a different directory structure). You may have to tell digikam on the main machine to look for new images.
I prefer not to mess with the database directly, as an error could make me lose the whole database…
For syncing over different machines: I’d again use sidecars for that: you can copy the changed sidecar files to the other machine and tell digikam to re-read the metadata for those images. Afaik, that is not done automatically.
To be safe, I do all such copying operations while digikam is not running.