How to Remove 20,000 Unwanted Pages from Google Index Effectively?

My website has approximately 20,000 unwanted .htm pages that have been indexed by Google. My goal is to completely remove these pages from the Google index and ensure they no longer appear in SERPs. So far, I’ve added Disallow: /*.htm$ to the robots.txt file, configured Nginx to return a 410 (Gone) status for .htm pages, submitted removal requests via Google Search Console, and cleaned up internal links using Rank Math. Additionally, I’ve removed these pages from the sitemap and submitted an updated version. Despite these efforts, some of these pages still appear in SERPs, and the bulk removal process in Search Console is not feasible for such a large volume. I’m looking for more effective and efficient methods to resolve this issue quickly and permanently. Any advice would be greatly appreciated. Thank you!