Can't send from custom domain email

I have an iCloud account with the primary email from GMail. I've registered an iCloud email address with it, but it's new. I've added a custom domain and am able to receive mails on the registered custom domain addresses. I can't see either of them in the "from" field when sending mail.

This is on both my MacBook M1 Pro Ventura 13.4 and my iPhone. Will try my iPad next.

I've waited a day and no change. My next guess is to change my Apple ID email address to the iCloud address above, but don't want to unless it's the reason why I can't do the above.

This is the same as experienced here, except my Mac is already updated:

Custom email domain: email not available … - Apple Community

Any ideas? Thank you!