Fix WordPress Sitemap Indexing Issues with Google Search Console

I am looking for a WordPress expert who can help me fix the sitemap indexing issues I am experiencing with Google Search Console. The website in question is around 4-5 years old with 8000+ articles. It was performing very well before we migrated to WordPress.com in early September (so previously website has www. , now has no www. due to WordPress.com policy).

Issues:
- Sitemap not indexed
- Couldn't fetch sitemap
- Sitemap could not be read

Steps taken:
- Verified sitemap using online tools
- Changed from Yoast to RankMath
- Flushed sitemap data to generate sitemap
- Edited robots.txt to follow standards
- Changed to custom sitemap plugin (disabled RankMath sitemap module)
- Resubmitted the sitemap, resubmit basic sitemap with only a few URLs

Ideal skills and experience:
- Strong knowledge and experience with WordPress and Google Search Console
- Expertise in troubleshooting and resolving sitemap indexing issues
- Familiarity with server settings and URL errors
- Ability to effectively communicate and provide clear instructions for resolving the issues