You’re definitely on the right track with how you’ve approached the migration—mapping Dropbox folders to SharePoint document libraries is a smart move. But yes, SharePoint's permission model can feel a bit more complex compared to Dropbox’s simplicity.
Creating a new site for every shared folder might sound appealing for easier access and permission control, but it can quickly become unmanageable at scale. Instead, sticking with your current approach—using document libraries and breaking inheritance—is more scalable and keeps your environment cleaner. To streamline the setup, consider using PowerShell scripts or third-party tools to automate group assignments and permissions.
Also, if performance is an issue with the default Dropbox to SharePoint Migration Tool, you might want to try the Kernel Dropbox to SharePoint migration tool. It’s a powerful alternative that simplifies Dropbox to SharePoint migrations, handles large volumes faster, and preserves structure, metadata, and permissions, saving time and frustration.