If it helps anyone, for the moments where the SSH console remains static and those moments when the cold panic sweats start to kick in… 
I ran this in another terminal to keep an eye on progress:
watch -n 10 'df -h /; echo; du -sh /var/discourse/shared/standalone/postgres_data* 2>/dev/null'Which updates you every 10s and keeps you sane
My 35GB migration took around 10 minutes.