RichText does not style text as expected

What you can do now is much simpler.

If you want to create a RichText with default TextStyle, just use Text.rich:

Text.rich(
  TextSpan(
    children: [
      TextSpan(text: 'Hello '),
      TextSpan(
        text: 'bold',
        style: TextStyle(fontWeight: FontWeight.bold),
      ),
      TextSpan(text: ' world!'),
    ],
  ),
)