As a workaround to break this cycle, you do not need to be signed in to use the official Microsoft billing support channels. I recommend visiting the main Microsoft Support website and navigating to the "Contact Us" section. From there, you can select the relevant product (such as "Microsoft 365" for commercial accounts) and describe your issue as "Billing and Subscription" related. When you speak with an agent, explain that you are the sole administrator of a work account, are locked out by 2FA, and need to either recover the account or cancel the subscription. Have your invoice and payment details ready, as you mentioned, to verify your identity and ownership.
Concurrently, please visit the Microsoft Account Recovery form at https://account.live.com/acsr. Since you know your password and have access to your email for notifications, this form is your best automated option for regaining access by proving ownership through alternative means.
This two-pronged approach—contacting billing support by phone to stop the charges and using the recovery form to regain access—is the most effective path forward. I am confident the support team will be able to assist you once you can speak with them directly.
I hope this guidance helps you resolve this stressful situation promptly. If this answer points you in the right direction, please feel free to mark it as "Accepted" 🙂.