I have not set a database user and password. I only added an encryption password to the database when creating it.
Please clarify what you are referring to here.
DT fills up my system SSD instead of saving the database to the chosen location.
Again, DEVONthink is not doing simple file copying. It’s not just passing data from point A to point B. It is receiving raw sync data locally, data it has to qualify, before passing it into the database. That raw data is going to be stored in DEVONthink’s Application Support folder while it’s processing. And your home directory is likely on the internal boot drive – and ideally should stay there IMHO.