I’m playing without DLCs, just the current version of the base game.
I have some slave population which are all at the bronze stratum. And some jobs of silver stratum that are free. However, none of the slave pops is upgrading from bronze to silver stratum. Is this somehow impossible? The slaves are from various enemy species, capable of FTL.
The type of slavery dictates what jobs they can do. If you go to the list of species in your empire (F6, I believe), you can read through the types of slavery and what work they allow, and then choose what suites you