Future readers: I will update this answer based on further research if I find a better way to do it, but this represents the current best effort. If you are reading this, and curious if I found a better solution, look below, if I found it i updated this answer, if I didn’t.. then I didn’t.
Currently the Font size picker component inserts the default font size internally:
function getSelectOptions( optionsArray, disableCustomFontSizes ) {
if ( disableCustomFontSizes && ! optionsArray.length ) {
return null;
}
optionsArray = [
{ slug: DEFAULT_FONT_SIZE, name: __( 'Default' ) },
...optionsArray,
...( disableCustomFontSizes
? []
: [ { slug: CUSTOM_FONT_SIZE, name: __( 'Custom' ) } ] ),
];
So default is always the first option, and there is no prop or filter to remove it. You could try to intercept it via the localisation API to rename it, but this would also change other controls.
To eliminate, move, or rename this option, you will need to do a hard fork of the FontSizePicker and TypographyPanel components, aka remove them and replace them with your own version. You can do this by disabling typography support in the editors data store via useIsFontSizeDisabled:
const fontSizes = useEditorFeature( 'typography.fontSizes' );
Turning off that feature then registering your own panel that has your version may work. However you will need to update it as new releases appear, it will need more work for Global styles when they arrive in WP 5.8, and you may need to fork other components to get around them self-disabling or changed functionality due to editor features being turned off at runtime.