That’s what I wanted to do… just wondering the exact steps you took as I tried it several times yesterday… both from my iPad and iPhone running 18.1
It would allow me to set my alias as the primary email… (and it would show my Alias email as my Primary Apple Account email), however…. Mail stopped working correctly both in the mail app, and iCloud Webmail. It wouldn’t let me delete, send or open emails. With various error messages coming up…
When I looked at the mail settings it was still showing my original iCloud email as my primary (so conflicted with what my Apple account was showing).
Apple support said others reported the same.
I’m wondering, as my original iCloud email also has a
@me.com email whether that is causing issues?
The only way to get mail working again was to revert the Primary back to my normal iCloud email (not the alias).