Sitemap cache invalidation help?

so, my current setup is pretty basic. i have a small saas app, and i'm using a custom PHP script to generate my sitemap.xml on the fly. it's served via Nginx, which i've configured with some pretty aggressive caching policies, mostly for static assets and general pages to keep things snappy. this includes the sitemap.xml file too, i guess.

the core problem i've been noticing is that even after i update content on my site – let's say a new blog post goes live or i update a product page description – the `lastmod` date in my sitemap doesn't update for hours, sometimes even a whole day. i've pretty much figured out this is because of Nginx caching the sitemap.xml itself. it's like google is seeing old information about when my content was last changed, which i know isn't good for crawling efficiency. it kinda defeats the purpose of having dynamic sitemaps if the updates aren't reflected quickly, right?

i've tried a few things, but none of them feel like a proper solution. first, i tried manually clearing the Nginx cache for the sitemap path. that works, the `lastmod` updates right away, but i can't do this every single time i update content; it's just not scalable at all. then, i thought maybe shortening the cache expiry for just the sitemap.xml. i tried setting a very low cache time, like 5 minutes, but then i started worrying about server load, especially if search engine crawlers hit it frequently. i don't want to accidentally DDOS myself or something. i also tried setting up a cron job to rebuild the sitemap every hour, thinking that would force an update, but then i realized that doesn't actually clear the Nginx cache for the existing sitemap, so it's kinda pointless. it just rebuilds the file in the background, but Nginx keeps serving the old cached version.

my main question is, how do experienced folks handle `cache invalidation` for dynamic sitemaps? specifically with Nginx or similar web servers. is there a programmatic way to tell Nginx to "bust" the cache for *just* the sitemap.xml file right after a content update happens in my app? or is there a better, more standard approach for ensuring the `lastmod` is always fresh and accurate without hammering the server with constant rebuilds or uncached requests? i'm really trying to understand the best practices for sitemap caching here.

any tips on server configuration, specific headers i should be looking at, or general best practices for this kind of scenario would be absolutely amazing. help a brother out please...