Custom domain error: Someone else in your Family Sharing group already started setting up this domain with iCloud

Has anybody received and resolved this error message when trying to move a custom TLD to your iCloud account: 

"Someone else in your Family Sharing group already started setting up this domain with iCloud"?

Summarized Background:

I'm trying to migrate a custom domain that I've owed and used for email for 10+ years from Host Gator to iCloud+. Here are the relevant details on my setup:

• Primary iCloud/Apple account with xxxx@me.com email address that have had for many years. This is the account to which I would like to migrate my domain for email hosting.
• Old Apple ID (created in iTunes days, pre-iCloud, pre Family Sharing, used for purchases), assigned to me, that had receipts@custom.tld as the primary email address. I have gradually stopped using this account since Apple "merged" Apple ID with iCloud--easier said than done.
• Family sharing set up for my 4 family members' iCloud accounts, and a non-person shared iCloud account
• One of my kids had hisname@custom.tld as an email (not primary) associated his Apple ID
• I attempted the step in iCloud+ to "Add a domain you own" and received the above error. So I assumed the blocking issue was that custom.tld was already associated with family's Apple IDs.

Steps taken so far:

• Created a new email Inbox for the Old Apple ID
• Added the the Old Apple ID to family sharing so that I now have 6 iCloud accounts in the "Family"
• Removed receipts@custom.tld from the Old Apple ID and assigned an associated Gmail account as the primary ID/email for this account. This process took about 5 days, with multiple layers of MFA on various devices and a LONG chat and 2 phone calls with Apple Support. I'll skip the long details.
• Logged the Old Apple ID out of a few infrequently used old Apple devices, that were still showing receipts@custom.tld as the account for Media/Purchases and/or deauthorized the old devices from that Apple ID.
• Removed hisname@custom.tld from my son's Apple ID

Current State

• It's been about 9 days since we removed hisname@custom.tld from my son's Apple ID
• It's been 6 days since I successfully removed receipts@custom.tld from my Old Apple ID
• Now, custom.tld should no longer be associated with any email addresses on my family's Apple accounts
• I am still getting the same error message.
• I have tried to move the custom.tld domain to my Old Apple ID/iCloud account
• I have also tried to move the custom.tld domain to my main iCloud account

I get the same error regardless of which of my two iCloud accounts I try to use to "host" the domain.

Any thoughts?

Do I just need to wait for timers on Apple's security steps to elapse? Or is there something else going on?

Will Apple block me from using the specific receipts@custom.tld email address, even if I am able to reassign the domain to iCloud, as some people are experiencing? Or has this policy changed?

On a side note, I own another custom TLD that never had email set up. I was able to add that domain to my main iCloud quickly and easily with no issues.

I have another call with Apple support scheduled for tomorrow.