I got a 404 page for the sitemap 😁.

That should work if you open the link in a new tab or in a private browser window.

Can I check what happens with posts published through peakd.com

On peakd.com we only pre-render some SEO critical attributes, not the whole post content. I don't think that has an impact on indexing as Google explained multiple times they will index the content anyway after rendering the client-side javascript.