Realistically I probably won't be filling one quickly, even though I'll be shooting RAW+JPG but it would be good to be able to backup everything each day.

Had a play with ... https://github.com/dmpop/little-backup-box ... on my Pi Zero W and it works nicely.

A few tweaks and adding lighttpd, h5ai (html file browser), samba, and eventually hostapd I think it will be a really nice little setup.