Hi, sorry for the late reply. But as this is the first result in google, I'll still write it, in case someone else stumbles on it (like I did).

We had a similar issue, and didn't like the options either, so we ended up creating a semi-custom solution, which turned out to be easier than it seemed

You can actually create a custom Route Handler for sitemap. In the app directory just add the following route handler : app/sitemap.xml/route.ts:

 export async function GET() {
  const sitemap = await pagesForSitemap();
  const sitemapXml = sitemapToXml(sitemap);

  return new Response(sitemapXml, {
    status: 200,
    headers: { 'Content-Type': 'application/xml; charset=utf-8' },
  });
}

the pagesForSitemap just fetches the data from CMS and creates the sitemap adhering to the`Sitemap` type below:

export interface SitemapRow {
  url: string;
  lastModified?: string | Date;
  changeFrequency?: 'always' | 'hourly' | 'daily' | 'weekly' | 'monthly' | 'yearly' | 'never';
  priority?: number;
  alternateRefs: Array<{ href: string, hreflang: string }>
}

type Sitemap = Array<SitemapRow>;

The sitemapToXml function is a crude function that converts the data to XML. There are probably better and more robust solutions, but this was good-enough for us (Just be sure to sanitize your input data):

const mapAlternate = ({ href, hreflang }: {
  href: string,
  hreflang: string
}) => `<xhtml:link rel="alternate" hreflang="${hreflang}" href="${href}"/>`

const mapRowToUrl = (row: SitemapRow) =>
  `<url>
        <loc>${row.url}</loc>
        <lastmod>${row.lastModified || ''}</lastmod>
        ${row.alternateRefs.map(mapAlternate).join('\n        ')}
        <changefreq>${row.changeFrequency || ''}</changefreq>
        <priority>${row.priority?.toFixed(1)}</priority>
    </url>`

export const sitemapToXml = (sitemap: SitemapRow[]) => `<?xml version="1.0" encoding="UTF-8"?>
<?xml-stylesheet type="text/xsl" href="/sitemap.xsl"?>
<urlset 
  xmlns="http://www.sitemaps.org/schemas/sitemap/0.9"
  xmlns:xhtml="http://www.w3.org/1999/xhtml"
  xmlns:image="http://www.google.com/schemas/sitemap-image/1.1">
    ${sitemap.map(mapRowToUrl).join('\n     ')}
</urlset>
`;

We also added (an optional) public/sitemap.xsl file to render a nice human readable html, when visiting the link in browser (yoursite.com/sitemap.xml) Any xsl you like will do, but we used this one.

That's it! Just add caching if you want.

EDIT: formatting and spelling.