@rolle Yes, that should do it.
Since you're in, what seems to be, a single user instance you could do the following:
1. Change the Mastodon config to point to the S3 (yes, it would break the existing downloaded images but if it's just you using the instance, it's not a big issue. Byt doing this the newly found image would start to be uploaded directly to S3)
2. Start the sync using the CLI so the already downloaded go to the S3.
3. Done?