The db file is referenced from another db file, so I suppose just copying it is not enough and that's what I never saw properly explained anywhere (and I have no idea how to test if the updated db file is being used). When doing the update directly from Garmin, other db files are changed to reflect the new db file name and other properties.

Having said that, if using all the sid files, placed in the proper place, I do get traffictrends and instasearch working properly so I don't even bother with the db file.