Problems with xml sitemap

1) Although the urls look correct (and now with https not http), the dates the files were last modified is before the date of my web migration.

The last modified date is stored in the WordPress database. Depending on how the migration added content from the old site, it’s possible that the last modified date was migrated from the old site.

2) Many of the page urls don’t appear to have images, but I have images on nearly every page

By default, our plugin does not analyze content that is outside of the main content box of a page or post. Additionally, the image URLs must be on the WordPress site. So if you migrated content and the image URLs are still from the old site, they may not be recongnized.

3) I am getting many strange 404 errors and think that google bots are still reading my old files in a previous sitemap and not finding them

This is normal when migrating a site. Google will eventually drop content that returns a 404. However, if the content migrated to a different URL format, you’re best option is to add redirects from the old to new format.

4) I can’t see an ‘XML sitemaps’ link in my Yoast menu on my WordPress dashboard to change anything which is strange as all my online research suggests I should be able to see this.

The XML sitemap menu item was moved to a new location a while back. Any tutorials that mention this menu are not using the current process. This article will help you enable sitemaps (which is done by default) and provide more information on how to customize them.
https://kb.yoast.com/kb/enable-xml-sitemaps-in-the-wordpress-seo-plugin/