Problems with verifying email for Apple ID

All, I know there was a thread on this 2 years ago, but I tried resolutions provided there with no success. Here (briefly) is the issue:

1) I try to log in to my AppleID online at appleid.apple.com

2) It asks me to provide my email and password associated with the apple ID (my email is a "@mac.com" address which I have used for at least 10 years)

3) It produces a 2FA request which I enter using the 6-digit code provided on another device

4) It then asks me to verify my and has auto-populated the "@mac.com" address I always use

5) I go ahead and enter my Apple ID password

6) It tells me 'this email is unavailable - please try a different email'

Things I have done:

• Tried using a "@me.com" email (which is also associated with the account)
• Tried using other emails I have
• Tried changing my Apple ID password
• Tried wiping browser history and using private browsing
• Tried on all my Apple devices (phone, tablet, mac)
• Ensured all OS updates (iPhone and Mac) are applied

I have been able to log into AppleID many times before successfully with this same email and generated app-specific passwords as well from the account.

I called Apple support and they worked with me for 45 minutes, then again a week later and could get no resolution. They said the next step is to take it (phone or another device)to a physical Apple Store, replicate the problem and see what to do. I plan to do that this week, but wanted to throw it out to the community to see if anyone else is having this issue, has found a solution or has anything else. I have been unable to sync my third-party calendar app (Fantastical 2) because I cannot generate a new app-specific password (which I needed to do when I 'reinstalled' the app on my new iPhone 14 Pro Max) and things seems to be going wonky (cannot create a shared album in Photos anymore either.

Thanks in advance for any help you might be able to provide!