So my company decided to migrate office suite and email etc to Microsoft365. Whatever. But for 2FA login they decided to disable the option to choose "any authenticator" and force Microsoft Authenticator on the (private) phones of both employees and volunteers. Is there any valid reason why they would do this, like it's demonstrably safer? Or is this a battle I can pick to shield myself a little from MS?
You have the right not to use your personal hardware for work, and the employer must provide the necessary equipment to accomplish your job.
Ask if you could get a hardware token (ie: Yubikey Security Key) instead of using Microsoft Authenticator to fulfill the security requirements. It's low cost and doesn't require a subscription unlike a cellphone plan.
Employers’ Responsibilities Towards
These are the main ones:
Employers must give their employees a place to work and make sure they have access to it. They must give them the tools, equipment and other things they need to do their work.