At first glance, I thought that several answers are somehow aside the point. Then I realized that my viewpoint was more specific than the original question. I already had created a separate job that backs up only the local Duplicati databases, but the original question seems to suggest the databases would be included in other jobs.
I came here to see if someone already has presented the idea of Duplicati providing a template of a separate backup job for its local databases. It really needs to be a template, not a ready-defined job, because there are many details each user may want to tailor to their own needs, such as which particular target destination and which schedule would the job have.
Once we limit the focus on such a separate backup job, many arguments here become moot. For one, the argument about the size of the local databases being too big is moot, because you only ever need one version of each in the destination. Also, since Duplicati does not run the jobs in parallel, the only local database that is being active during this separate job’s run is its own database, which need not be included the set anyway. When a disaster happens, and the local databases need to be restored from this set, you do not have access to any database anyway, so you have to let Duplicati rebuild the database for this separate job first. And then, if Duplicati provides the template, you can set it up easily again.
So how about it, could you consider providing this kind of a template?