The plugin’s sitemap was simply specified in robots.txt:
Sitemap: https://site.com/wp-sitemap.xml
For some reason, Google indexed this site really briskly, but I’m not seeing this happening en masse yet.
Each sitemap contains 2k URLs. No need to add more. The standard WP sitemap without plugins is the fastest, in my opinion.
Here’s the second indexing method.
Since you guys were asking in the comments.
And the third method—You just need to ping it (c)