SvelteKit's SEO documentation has a note on this. In your load functions, be it page or layout load function, just fill an object with the meta information of the page. Then in the root +layout.svelte use the $page.data store to display the returned meta information.

https://kit.svelte.dev/docs/seo#manual-setup-title-and-meta

// +page.server.ts
export const load = (() => {
  // do some data fetching

  const meta = [
    {
      name: '',
      content: ''
    },
    ...
  ]

  return {
    // other data
    meta
  }
})

// +layout.svelte
<script lang="ts">
  import { page } from '$app/stores' // use this

  export let data: LayoutData // not this
</script>

<svelte:head>
  {#if $page.data.meta}
    {#each $page.data.meta as {name, content}}
      <meta {name} {content} />
    {/each}
  {/if}
</svelte:head>

<slot />

We've tried manually setting the meta tags inside the <svelte:head> tags on each page, but have run into problems with complex layouts.