URGENT: Free XML Sitemap Generator Failing on Large Sites with technical SEO Error

I'm completely stuck and desperately need some help with our Free XML Sitemap Generator. It's consistently failing for larger websites, causing major technical SEO headaches for our users. While it works fine for smaller sites with a few hundred URLs, anything over approximately 500 URLs just breaks. The generator either times out or throws a generic server error after processing a certain number of URLs, which is directly preventing proper website indexing for our users.

I've been trying to debug this for hours and I'm seeing a recurring error in the logs that looks something like this:

[2023-10-27 14:35:01] production.ERROR: Maximum execution time of 300 seconds exceeded for SitemapGenerator.php on line 125. {"exception":"[object] (Symfony\\Component\\ErrorHandler\\Error\\FatalError(code: 0): Maximum execution time of 300 seconds exceeded at /var/www/html/app/Services/SitemapGenerator.php:125)"}

Here's what I've attempted so far to fix this:

• Increased PHP max_execution_time to 600 seconds.
• Optimized database queries for URL fetching to speed up data retrieval.
• Tried processing URLs in smaller batches, which was partially successful but still times out on the final merge step.

What could be causing this persistent timeout on larger sites, and what advanced strategies can I implement to make our Free XML Sitemap Generator robust enough for thousands of URLs without hitting execution limits?