Astra Theme is a very popular user choice theme for WordPress users with over a million downloads. If you checked in lately with the WordPress free repository, you might have noticed it did not show instead an alternate Astral theme showed.
There are various ways to add your website to google. Out of all the ways adding your website to google search console via add a domain property is the best one. You would not have to reverify your website again and it covers all the prefixes and pages of your website automatically. Its a one-time effort but worth the pain. It isn't tough and I have provided a step by step guide.
To change the admin username without deleting and creating any new user, you would need to login to your cpanel of the hosting account and then you need to follow the steps given here.
Yoast SEO plugin is provided by the Yoast team which has been in this game for long. Yoast SEO plugin helps you to manage your SEO easily. This is one of the most popular plugins along with RankMath.
To be seen on Google you need to add your WordPress website to the Google Search Console. What this means is that you submit your website to Google and google bots start crawling on your website. The bots go through every possible link on your website and index your entire website as it stores it on Google servers.