This question already has an answer here:
I am trying to figure out the best way to organize a Marketing Cloud instance. I want to get the best way to do so as the Multi-org is not an option (Data streams and Journey Builder do not work) and there is no other way to connect one BU to Salesforce production environment and another BU to sandbox environment.
The only way I can think about is to create a BU for testing, create testing data that replicates the data in the production BU and when everything is as expected, copy one by one all the contents, filters, emails, queries, automations... to the production. With emails, I could share them to the production environment so all the content resides in the test BU.
Any clue if this is the only way to maintain a test and production environments in Marketing Cloud or if there is another workaround?