Revised 31 October 2020
In the past I’ve recommended various ways to produce a sitemap.xml for free but all of them ceased to work after a time. However, I do now have a solution which has worked well for about a year. See:
This allow you to make three large sitemaps that work perfectly.
For WordPress I’ve installed a plugin which provides a sophisticated sitemap.xml system which seems to be working well (google-sitemap-generator.4.1.0).