How to properly type tm, rt functions?

[ConorMurphy21]

I have some code that looks like this

// Example.vue
<template>
  <div v-for="section in $tm('foo')">
    <h1 :id="section.key">{{ $rt(section.header) }}</h1>
    <p>{{ section.body }}</p>
  </div>
</template>

<script lang="ts">
import { defineComponent } from "vue";
export default defineComponent({});
</script>

// en.js
type Section = {
    key: string,
    header: string,
    body: string,
}

export default {
    foo: [
        {
            key: '1',
            header: '1',
            body: '111',
        },
        {
            key: '2',
            header: '2',
            body: '222',
        }
    ] as Section[],
}

My goal is to get this code to work, preferably using the global methods with the options API.
I've tried adding the global type declaration:

// vue-i18n.d.ts
import {
  DefineLocaleMessage,
  DefineDateTimeFormat,
  DefineNumberFormat
} from 'vue-i18n'
import en from "@/locales/en";

declare module 'vue-i18n' {
  // define the locale messages schema
  export interface DefineLocaleMessage extends (typeof en) {}

  // define the datetime format schema
  export interface DefineDateTimeFormat {}

  // define the number format schema
  export interface DefineNumberFormat {}
}

I've also tried using the composition style useI18n<{message: typeof en}>. But I keep getting: 'key does not exist on type 'never' (when using global $tm) and 'number' when using useI18n.
Any help is much appreciated!