Using sitemap index for post sitemaps

According to sitemap, each sitemap file should not contain more than 50,000 urls. Ours contains 57,000 at the minute

Ideally, we’d like to have multiple sitemap files for our posts, with one main index file (sitemaps.org - Protocol), but I’m unable to find any information about doing this with Ghost.

One Option is to create a custom sitemap template (Tutorial: How to create a Google News Sitemap with Ghost), and have a limit of x amount of articles, but this is also not ideal as it means we’ll be missing out lots of the articles.

Is there something we can try that would fit the usecase?