500 can be caused due to these ->
Corrupted WP core files
Corrupted .htaccess files
Problematic plugins
PHP memory limit issues
Problematic theme
As you already
- I have re-copied the .htaccess file and put in what WordPress recommended.
- I have stripped the plugins from the site through FTP.
I think
Upload fresh copy of wp-admin and wp-includes to your WordPress site
Can help.
This article has other solutions too, Please do check
https://www.navthemes.com/http-error-500-wordpress/