Hello Steve TM
First of all, I would like to clarify one point: this forum is intended for users to assist one another. No one, including myself, has the authority or the tools to unlock MFA on your behalf.
About your questions:
1> Correct route?
For this type of issue, the Microsoft Data Protection Team is the only team that can assist further. Contacting them directly is the only available way to regain access to the tenant.
You can follow "Steps 1 and 2" below to contact the Data Protection Team so they can verify your information and assist you in regaining access to your account.
2> Is DNS TXT verification accepted?
Yes. DNS TXT verification is commonly used, but Microsoft may also request:
- Tenant ID
- Billing/subscription details
- Proof the requester is authorized (e.g., company director)
1/ Contacting Microsoft Data Protection team by phone support
In this situation, the Microsoft Data Protection team has tools and processes in place to verify identity and regain access to administrator accounts.
Therefore, if you are the only administrator in your organization, then you need to involve Microsoft data protection team. Please try to find the related hotline number to call the frontline let them raise a ticket for you: Customer service phone numbers - Microsoft Support
If you can use English, you may prefer these numbers:


(Important Note: Depending on your country or region, when you call the support number, you may hear an introduction of about 30 seconds such as "you can visit the link...". You can ignore this introduction and wait until you are presented with the options. Then press "1" as a business email user, and again "1" for technical help.)
In some countries, this is an automated conversation: First, when you call the hotline, they will ask you what kind of problem you are struggling with.
Answer: Authenticator.
A: What products do you use?
B: Office 365 for business.
Verification: Education or company account?
B: For companies
A: Are you an administrator?
B: Yes.
A: Are there any other administrators in your organization?
B: No.
A: I need one.... Service request?
B: Yes
If your organization's Office 365 Business subscription is from a partner or reseller, and the global administrator is unable to open a service request on your end, contact the reseller's support provider to help open a service request on behalf of you instead.
Please try "2 or 3"times until you can reach out to an agent.
Besides, if you still cannot reach to agent, there is still a workaround, you might consider registering for a new tenant by signing up for a trial subscription. This would allow you to create a new tenant following the prompts provided. Once set up, you can access the admin console of the new tenant and submit a support ticket requesting to speak with the Data Protection team on behalf of your previous tenant.
Another work around
(In my experience, this approach has a higher likelihood of success than contacting phone support. To make the process a little easier, I have also included all the relevant information and guidance I have gathered below. You may want to give it a try and see whether it helps in your situation )
For detailed guidance, to set up a new tenant, please follow these steps:

Once your tenant is created, you should be able to access the support portal and submit your ticket without further issues.
Ticket Support: In the Microsoft 365 Admin Center->Support->Help & Support. You can raise support ticket In Microsoft Admin Center by this link:
https://admin.microsoft.com/#/support/requests


Tips: You can switch off the Support Assistant as shown in the screenshot, then type "Contact Support" into the search box and select "Contact Support" or the "earphone symbol" above..
- Choose "Phone" support and fill all the details of this issue.
- Click "Contact me".
After completing them, you will receive an email appointment for a meeting to review this issue professionally together.
*Please remember to cancel the trial subscription once your issue is resolved to avoid any unintended charges.
Wish you all the best!