Topic says it. Add DbTable::setPrefix() and a property $prefix to match. Then, remove all of the logic that looks for it in the methods. Then, you can remove all of that from the install too.
While I am here, there should also be a separate method for each of those functions to get the query. This way you can test what the query is. For instance DbTable::getCreateTableQuery(). The MySQL and Postgres classes would not implement db_create_table(), just getCreateTableQuery(). Of course, each of the current methods would just be { return $this->db->query('', get*Query(), ['security_override' = true]); }
/me keeps this unread.