So I have resolved my issue. In case someone else comes across this....
sgdisk --zap-all /dev/sdd did NOT remove all GPT metadata. I had to run gdisk /dev/sdd, select x (Expert) and select z (Zap). I was then able to create directory and add Datastore via UI.
So I'm having a strange issue after following steps in #6 above. I removed a PBS datastore (named "backup") and now trying to recreate - I was testing creation of an encrypted destination for PVE backups.
I have removed the existing datastore:
systemctl disable...