Back up my whole Dropbox to Google Cloud Storage using Dropbox webhooks.
Some existential worry that Dropbox could blow up someday, but also it's relatively cheap to store data in the Archive storage class (I have about 400 GB, so should be about $0.48/month).
Make a Dropbox app. You will only need to process your own files, so it can stay in Development status.
This process is split into three Google Cloud Functions, each with a folder in the repo root:
webhook-- respond to Dropbox webhook. In your Dropbox app, set the webhook URL to the URL of this function.queue_files-- triggered by the previous job. Figure out the last cursor. Directly delete any files that have been deleted from Dropbox. For new files, queue them to the new job.process_file-- download the file from Dropbox and upload it to GCS.