Hello @Solias99,
Your _id should be ObjectID, it’s very recommended. It offers features and is very well managed by Mongo (and Forest). https://docs.mongodb.com/manual/reference/method/ObjectId/
Further of that, let’s consider your “customers” and “subscriptions” collections.
-The customer schema is good (but where are the other fields, they should be together here)
-The data extract you showed in your last screenshot indicates in a correct way a customer document having many subscriptions.
Please try to generate your schema locally (with a local Mongo) before deploying it, if it doesn’t work manually.
Regards