That’s not going to help anyone.

The Five Eyes is an Anglosphere intelligence alliance comprising Australia, Canada, New Zealand, the United Kingdom, and the United States. These countries are party to the multilateral UKUSA Agreement, a treaty for joint cooperation in signals intelligence.

https://en.wikipedia.org/wiki/Five_Eyes

Even being stored in EU doesn't preclude your data from being targeted by signals intelligence. Which is different than requiring US based companies to provide non-US data to American government.

Does fastmail have a US presence? If no - then they're not bound at all by US jurisdiction.