Niker

Проще всего сделать так, ts сам подберёт типы:

const elementDefaults = {
  'welcome': {
    placeholder: 'Type welcome message...',
    question: '',
    isDescription: false,
    description: '',
    startButtonText: 'Start',
  },
  'checkbox': {
    placeholder: 'Type question or statement...',
    question: '',
    isDescription: false,
    description: '',
    options: [] as string[],
    multiple: false,
  }, 
  'dropdown': {
    placeholder: 'Type question here...',
    question: '',
    isDescription: false,
    description: '',
    options: [] as string[],
  },
  'rating': {
    placeholder: 'Type question...',
    question: '',
    isDescription: false,
    description: '',
    steps: 10,
    defaultRate: 0,
    shape: 'stars',
  },
  'text': {
    placeholder: 'Type question...',
    question: '',
    isDescription: false,
    description: '',
    maxLength: 99999,
  }, 
  'slider': {
    placeholder: 'Type question...',
    question: '',
    isDescription: false,
    description: '',
    steps: 10,
  }, 
  'thanks': {
    placeholder: 'Type message...',
    question: '',
    isDescription: false,
    description: '',
  }
}


export function getElementSettings<E extends keyof typeof elementTypes>(type: E) {
  if(!(type in elementDefaults)) throw new Error("element type doesn't match!");
  return elementDefaults[type];
}

Но если хочется по правилам, то как-то так:

type ElementSetting = {
  placeholder: string;
  question: string;
  isDescription: boolean;
  description: string;
}

type ElementSettings = {
  welcome: ElementSetting & {
    startButtonText: string;
  },
  checkbox: ElementSetting & {
    options: string[];
    multiple: boolean;
  },
  dropdown: ElementSetting & {
    options: string[];
  },
  rating: ElementSetting & {
    steps: number;
    defaultRate: number;
    shape: string;
  },
  text: ElementSetting & {
    maxLength: number;
  },
  slider: ElementSetting & {
    steps: number;
  },
  thanks: ElementSetting
}

const elementDefaults: ElementSettings = {
  'welcome': {
    placeholder: 'Type welcome message...',
    question: '',
    isDescription: false,
    description: '',
    startButtonText: 'Start',
  },
  'checkbox': {
    placeholder: 'Type question or statement...',
    question: '',
    isDescription: false,
    description: '',
    options: [],
    multiple: false,
  },
  'dropdown': {
    placeholder: 'Type question here...',
    question: '',
    isDescription: false,
    description: '',
    options: [],
  },
  'rating': {
    placeholder: 'Type question...',
    question: '',
    isDescription: false,
    description: '',
    steps: 10,
    defaultRate: 0,
    shape: 'stars',
  },
  'text': {
    placeholder: 'Type question...',
    question: '',
    isDescription: false,
    description: '',
    maxLength: 99999,
  },
  'slider': {
    placeholder: 'Type question...',
    question: '',
    isDescription: false,
    description: '',
    steps: 10,
  },
  'thanks': {
    placeholder: 'Type message...',
    question: '',
    isDescription: false,
    description: '',
  }
}


export function getElementSettings<E extends keyof ElementSettings>(type: E): ElementSettings[E] {
  if(!(type in elementDefaults)) throw new Error("element type doesn't match!");
  return elementDefaults[type];
}

const elementSettings = {
  welcome: {
    placeholder: 'Type welcome message...',
    question: '',
    isDescription: false,
    description: '',
    startButtonText: 'Start',
  },
  checkbox: {
    placeholder: 'Type question or statement...',
    question: '',
    isDescription: false,
    description: '',
    options: [] as string[],
    multiple: false,
  }, 
  dropdown: {
    placeholder: 'Type question here...',
    question: '',
    isDescription: false,
    description: '',
    options: [] as string[],
  },
  rating: {
    placeholder: 'Type question...',
    question: '',
    isDescription: false,
    description: '',
    steps: 10,
    defaultRate: 0,
    shape: 'stars',
  },
  text: {
    placeholder: 'Type question...',
    question: '',
    isDescription: false,
    description: '',
    maxLength: 99999,
  }, 
  slider: {
    placeholder: 'Type question...',
    question: '',
    isDescription: false,
    description: '',
    steps: 10,
  }, 
  thanks: {
    placeholder: 'Type message...',
    question: '',
    isDescription: false,
    description: '',
  }
} as const;

type Editable<T> = { -readonly [P in keyof T]: T[P]; };
type ElementSettings = typeof elementSettings;

export function getElementSettings<T extends keyof ElementSettings>(type: T): Editable<ElementSettings[T]> {
  if(type in elementSettings) {
    const settings = {...elementSettings[type]};
    if('options' in settings) {
      (settings as {options: string[]}).options = (settings as {options: string[]}).options.slice();
    }
    return settings;
  }
  throw new Error("element type doesn't match!");
}