Running site from directory FAIL – please help!

OK, I had an HTML site and a separate (old) WP blog under /blog.

I created a new directory /wp29 and installed fresh WordPress in there. I then:
1) recreated the old site as WP pages (check!)
2) exported all the old posts and imported them into the new site, and deleted the old blog. (check!)
3) backed up the new blog (check check!)
4) deleted the old wp install under /blog (check!)

So far so good.

Now I wish to make my new site display at the root level https://www.mydomain.com and the blog posts display at the https://www.mydomain.com/blog (FAIL!)

Following the directions of this page, https://codex.www.ads-software.com/Giving_WordPress_Its_Own_Directory , I did the following:

I changed the main WordPress URL to https://www.mydomain.com
I left the Blog URL at https://www.mydomain.com (didn’t think this would make a difference yet)
I saved. My blog disappeared as expected.
I moved index.php and .htaccess to the root directory
I edited index.php as directed, adding my directory name (“/wp29”) to the string
require(‘./wp29/wp-blog-header.php’);

As far as I can tell, that’s all I am _supposed_ to have to do. But now nothing works. The site is not displaying at https://www.mydomain.com/index.php. It displays without any formatting from https://www.mydomain.com/wp29/index.php, but I can’t login to make any changes because I get an error at https://www.mydomain.com/wp-admin as well as https://www.mydomain.com/wp29/wp-admin (the latter used to work).

So… what did I miss?

What can I do to at least get back to square one (besides installing from backup)?

Thanks if you can help!