Generate database diagram from SQL Server with no proper integrity
Try Dataedo - it's a database documentation tool that I think can help in your case. You will be able to import multiple database schemas into one repository, then further split them into smaller pieces called modules. You can describe with rich text and create ER diagram for each. You can define missing FKs and use them for the diagrams. You are able to describe every data and program element (table, column, function, parameter etc.) and export such a documentation to PDF document or HTML. Maybe this will straighten out your spaghetti ;)

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