It doesn't really matter whether you use simple
.txt sitemaps or XML sitemaps; Google supports both. Although XML sitemaps do offer greater control and may be easier to manage on a larger site, or if you have different types of content, and text-only sitemaps may not be supported by other search engines.
But as already mentioned in comments, unless a site is hard to crawl in the first place, a sitemap (XML or plain text) is not strictly necessary to get indexed by the search engines. It is simply advisory - a method to allow URL discovery. Even if URLs are contained in a sitemap is no guarantee that they will be indexed. The sitemap itself is not a ranking factor.
However, a sitemap does have other benefits when used with GSC, as Google will report on the index state of just the URLs contained in a sitemap. You could, for instance, have multiple sitemaps for different parts of the site and see how different parts of the site are indexed, rather than only reporting on the whole.