Sitemap Post Limit

Your plugin is awesome, I really appreciate your hard effort on developing such a plugin with the white label.

I have a small problem, I can tweak plugin myself but I didn’t want to mess up anything.

(Only the last 1000 items are listed in Sitemaps for performances issues.) I would like to remove this limitation.
Show 1000post on a post-sitemap1.xml then next 1000 in post-sitemap2.xml like that.

Recently I have moved a blog from blogger to WordPress, which is of nearly 6000post. I would like to show sitemap for them like Yoast sitemap does.

Thanks