In your current structure you have provided no way to assign a plant to a job site.
During the previous ERD discussion, it was suggested that you create a join table for this purpose.
Your structure does not contain the join table.
"NOTE: I am unsure if this is bad practice or simply unnecessary but I serialized the _kf fields in the join table."
It is bad practice.
It was previously discussed.
It CANNOT work.
The point of a foreign key is that it will contain values which come from the primary key of the parent table.