MongoDB is very flexible and it is a blessing and a curse. It will store any data you provide. By default there is no validition you setup validation rules on database or use ODM like Mongoose if you're using NodeJs to help with validation in the application layer.
What is serverless? Serverless is a service where you pay for how much computing power is used, not how much you reserve. Why migrate? Migrate reasons can be several: from security, to cost
Assign only those permissions which are required to get the job done. Also utilise infrastructure as a code (IaC) tool like Terraform to track changes in your infrastructure to prevent unwanted human error and if it happens track down easily.