Sitemap Cache Invalidation Woes?

Hey folks,

• Quick Recap: Following up on the lastmod timestamp discussion. My sitemap is still playing hide-and-seek with Google.
• The Problem: Even when my backend is correctly updating lastmod for new content, the sitemap served to Googlebot seems to be stuck in the past. It's like my server's got amnesia, refusing to acknowledge recent changes.
• Specific Frustration: New blog posts or product updates are taking hours, sometimes half a day, to appear in the sitemap. This totally defeats the purpose of dynamic sitemaps and makes me question my cache invalidation strategy (or lack thereof).
• My Ask: What are your go-to methods for ensuring sitemaps reflect changes instantly without hammering the server? Are there clever tricks for efficient cache invalidation for sitemaps?
• Closing: Anyone else's sitemap doing a slow-motion dance with their content updates?