You can try with data subsetting tools, a few that I know are: greenmask, jailer, tonic condenser, and I think there are more. They are not exactly for your use case, but might help you a bit. The problem is most of these tools can walk foreign keys, but only in one directions.

Also, since you said FK are not fully implemented, then there just won't be enough information for them to work with. So you can either work on fixing the foreign keys or have more complex scripts.

This is poor design. Multi-tenant data model design must have a tenant or segment ID for every tenant segmented tables. While it may not be a problem initially but as the business grows, you may need to move tenant data to separate DBs or even different regions based on compliance requirements. IMHO it is a good idea to run DB migrations and have a tenant ID populated in all tenant segmented tables