Which fields matter
Google has said it ignores changefreq and priority, and uses lastmod when it is consistently accurate. Bing also relies on lastmod. So the most useful thing in a sitemap, after the URLs, is a truthful last modified date per page.
If every page says daily and priority 1.0, the values carry no information. If lastmod is today on every URL, crawlers learn to distrust it.
Questions
Should I remove changefreq and priority?
You can. They do no harm, but they do not help with Google either.
How fresh should lastmod be?
As fresh as the page's real last meaningful change, no fresher.
What is the limit for one sitemap?
50,000 URLs and 50 MB uncompressed. Larger sites split into several files listed in a sitemap index.
Is my data stored?
No. Sitemaps are read or built for the result you see and not kept.