Fix WordPress 500 Error After Migration

I just migrated my WordPress install from a “/sub” folder to the root of the domain and immediately started getting a 500 Internal Server Error on every page, including wp-admin.

Here’s what I have already confirmed:
• File and folder permissions look fine.
• Site URL and Home URL are correctly updated in the database.
• The .htaccess file was adjusted for the new root location.
• I briefly disabled all plugins, renamed the active theme, and even replaced .htaccess with a vanilla set of rules—no luck.
• WP_DEBUG is on, yet no debug log is generated.

I need an experienced WordPress troubleshooter to dive in, locate the hidden cause, and bring the site back online in the root directory. Feel free to use WP-CLI, cPanel, SSH, or server error logs—whatever gets the job done swiftly and safely.

Acceptance criteria:
1. Front-end and wp-admin load normally from the root URL.
2. All previous content, plugins, and theme settings remain intact.
3. A brief summary of the fix is provided so I can avoid the same issue next time.