How can I delete original photos locally and retain the low-res versions?

Can you confirm that all of your photos and videos have uploaded to the cloud? Check https://www.icloud.com/#photos

Because it sounds like the error might be that they're only uploading if they first are copied from your external drive to the internal drive.

Assuming they have all uploaded, you can (after making a backup), simply delete your local Pictures > Photos Library.photoslibrary. Photos.app will create a new such file when you start it again, and only low-res versions of your images will be downloaded.

If you have not succeeded in uploading all of your photos yet, your best bet is probably to switch Photos Library.photoslibrary to be on your external drive, to avoid filling up your internal drive.