WordPress in subdirectory and htaccess

I have developed a WordPress site to replace my client’s old site. The development was done in a subfolder of the existing hosted domain (call it “wp”). When the site development was finished, I made the WordPress site live in that subdirectory by a) changing the site URL and the WordPress URL in the SETTINGS panel, and by copying the .htaccess and the index.php files from the wp folder into the root folder, and editing the index.php file to change to path to the wp-blog-header.php file so that it included the subdirectory name. This worked well, and the WordPress site is live and fully functional.

Now, having mounted a secure certificate on the site, I want to redirect all incoming fetches from the http protocol to the https protocol. This is something I have done successfully on nearly every site I have developed over the past several years. Yet on this site, when I add the necessary rewrite conditions and rules to accomplish the rewrite, any attempt to access the site results in “Too many redirects occurred trying to open…”. Support at the host told me that the “wp” directory is the problem. I’m sure I’ve done this before with no problem. Can you tell me what might be going on, or what additional info you need to troubleshoot this?

Thanks!