Question: With this method, what would you do if , in a few months, Microsoft flags the account and asks you to verify with a text SMS? (I'm aware if one sets up TOTP hopefully they wouldn't do this, but one never knows) - can you reactivate the SIM smoothly?

1. create account with phone #

2. add TOTP authentication

3. remove phone #

I think your parent comment is saying MS flags the account as questionable and asks you to confirm your current phone number (the one you already used a burner for).

Having TOTP enabled should ward off the "suspicious login" prompts that ask for your phone number. If they still want your phone number for whatever reason, you'll have no choice but to buy another burner phone, but there's no reason they wouldn't accept a new phone number. You did remove that phone number from your account, after all (people change their phone numbers all the time).

Any tips on SIMs? I'm only seeing $20 ones from Tracfone.