Adding bold/italics to string field without full block editor

Your current approach is actually pretty solid! Using an array with a single block type and validation to limit it to one line is the standard way to add bold/italic formatting to a title field in Sanity. However, there are a few things you can do to make it feel less like overkill:

Making it more title-like

According to the block type documentation, you can use the oneLine option to restrict the Portable Text input to a single line:

{
  name: 'title',
  title: 'Title',
  type: 'array',
  of: [
    {
      type: 'block',
      styles: [],  // Remove style dropdown (no H1, H2, etc)
      lists: [],   // Remove list options
      marks: {
        decorators: [
          {title: 'Strong', value: 'strong'},
          {title: 'Emphasis', value: 'em'}
        ],
        annotations: []  // Remove link option
      },
      options: {
        oneLine: true  // This prevents hitting return!
      }
    }
  ],
  validation: Rule => Rule.required().max(1)
}

The key improvements here:

1. oneLine: true - This disables the ability to hit return and keeps everything on a single line
2. styles: [] - Removes the style dropdown (Normal, H1-H6, blockquote)
3. lists: [] - Removes bullet/numbered list options
4. annotations: [] - Removes the link annotation if you don't need it

Styling the editor height

Unfortunately, there's no built-in option to reduce the WYSIWYG height to match a string field exactly. The Portable Text Editor has a minimum height by default. However, you can use custom components to add custom CSS:

{
  type: 'block',
  // ... other config
  components: {
    input: (props) => (
      <div style={{ minHeight: 'auto' }}>
        {props.renderDefault(props)}
      </div>
    )
  }
}

This approach gives you inline formatting in what feels much more like a title field, while still being technically correct as Portable Text.