How can I restrict/limit the permissions provided to SimpleBackups?

It depends on the backup type.

Set up a user with READ access on the folders that you need to back up, and just WRITE permission on the operations directory.

The operations directory is where SimpleBackups stores files required for its operation. Example, backup log during a running backup, rclone which is used to send backups to your storage.

Set up a user with READ access on the databases you need to back up.

You can give SimpleBackups as little permissions as possible on your storage. It depends on the storage, but just read/write to where backups should be stored it fine. You may also connect a read-only storage and do not provide SimpleBackups with a write permission if you prefer.