First of all choosing the subset of standards that Google chooses is a sovereignty problem. Gov services should not be constrained to what Google in the US decides to implement. Of the 3 browsers you mention, Chrome is subject to google snooping. Firefox is limited in Google’s influence as well. And Safari only serves Microsoft and Apple users officially.
The gov need not produce a browser from scratch, but they need to officially support a non-controversial browser that is not tied to US tech giants.