Hi @Elden Von , one possible reason could be related to Exchange Mail-Enabled Security Groups and distributed lists, as mentioned in the post you found. Since you cannot access the Exchange Admin Center (EAC) with a personal account, you might need to use an account with the appropriate permissions to access EAC and check for any lingering ProxyAddresses that rely on the custom domain name. These addresses may need to be updated manually to another domain name before you can successfully delete the domain.
Please let me know if you have any questions and I can help you further.
If this answer helps you please mark "Accept Answer" so other users can reference it.
Thank you,
James