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Something is not quite right in what you describe.

Joomla's .htaccess recommendations have not changed since Joomla! 5.1.

No changes have been made between 5.2.2 and 5.2.3 which affect URL routing.

You seem to have a problem displaying the sitemap –which is a simple .htaccess Maker option change– BUT you also claim that the .htaccess is irrelevant to your problem. I think that this part of your issue description is wrong.

All the SEO tools you mentioned use a SEF URL for the sitemap, usually /sitemap.xml. However, accessing files at the site's root is restricted to Joomla's index.php by default when you use the .htaccess Maker. Therefore, you need to add sitemap.xml to the “Allow direct access to these files” option in the .htaccess Maker. This allows search engines to view the sitemap. If you try to open it in a browser you will still get a 403. Fixing that requires adding the URL to the XSL file converting the sitemap XML into HTML; this varies by the tool you use.

If that sounds weird to you, take a look at my blog's sitemap (powered by JLSitemap) where I have done what I explained above.

So, for a first step I'd ask you to replace the contents of your .htaccess file with those from Joomla's recommended for Joomla! 5.2.3 without making any other change. Does that allow you to view your sitemap?

If changing to Joomla's .htaccess fixed access to the sitemap, follow the instructions above to add sitemap.xml to your .htaccess Maker configuration, and regenerate the .htaccess with the .htaccess Maker. Then, without changing anything, come back here so I can give you further instructions for the XSL file.

If changing to Joomla's .htaccess did not fix access to the sitemap, check in administrator, Components, Admin Tools for Joomla!, Web Application Firewall, Blocked Request Log for the IP address you accessed your sitemap from. I want you to tell me the Reason and Target URL listed there.