Hello, @fsdg,
Welcome to the Microsoft Q&A platform!
Based on your description, I understand that you want to add a new domain (TXT record) to the O365 portal without migrating MX, autodiscover, or SPF records ,and you want to know whether you need to change the registry key on all clients to prevent Outlook from connecting to Office 365.
According to Microsoft's official documentation, there are three ways to validate a domain after you add a new domain, which you can refer to the following screenshot. For more information about adding a new domain, please click on Add a domain to Microsoft 365 - Microsoft 365 admin | Microsoft Learn for reference.
During the process of adding a new domain, there is no “migrating MX, autodiscover, or SPF records” as you mentioned. Can you explain what it refers to? Does it refer to doing something with the new domain you added? If so could you please clarify your requirements? I hope you could get back to me in your free time, it will help to solve your problem faster.
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Thank you for your support and understanding.
Best Wishes,
Alex Zhang