Thanks @ppr and @Jon_Smith
It sure helps looking at a problem with fresh eyes… 
This wording made me believe the Code Source Files could only be “used” within Coded workflows, and not in “regular” workflows.
I’ve put my classes into Code Source Files now, imported the namespace into my “regular” workflow and deserialized directly in an assign.
It works as it should now.
Another error I got while trying to publish threw me off.
I already had a sequence named Vehicles elsewhere in my project, at the root folder
Deleting or renaming the sequence removed the error.