Deleting & reusing an iCloud alias

What I thought was my primary iCloud email address actually turned out to be an alias I had set up way back when. Because I've just added my iCloud email to Outlook for Windows, I had to set it up using my primary iCloud email and not the alias which is not what I wanted (as emails sent/received would be from a different email address to what I normally use).

I deleted the alias from my iCloud mail in the hopes this would make it available to use as a primary email for a new Apple ID I created specifically for this purpose. It does not appear to have worked though as when I try and create a new @icloud.com email address using the old alias name, it says it's not available.

Any ideas??