Laravel SEO sitemap indexing?

we're building out this dynamic xml sitemap solution for laravel, aiming for auto-updating and future-proof. it's been pretty solid for smaller sites, but now we're scaling it up to handle millions of urls, and we're hitting some snags with how google and other engines actually *proces* these massive, frequently updated sitemaps. the core issue isn't generating the sitemap itself – we've got that down with efficient chunking and caching. it's more about ensuring optimal `laravel seo` by getting these dynamic urls indexed quickly and reliably. specifically, we're seeing inconsistencies with `lastmod` updates being respected by crawlers, and sometimes a significant delay in new content appearing in SERPs even with sitemap submissions. we're using sitemap index files, but even then, the update frequency seems to be an issue.

what are the absolute best practices or advanced strategies for pushing high-volume, dynamic content through sitemaps to ensure near real-time indexing for `laravel seo`? especially concerning `changefreq` and `priority` for large sites, or maybe some hidden tricks with sitemap submission apis?