I would disable .htaccess and changing the permalink structure that can be found in (setting->permalinks).
You’re specifically getting a 500 error and not a “Cannot connect to the Database” error?
Since it sounds like a theme issue, inside of wp-config.php, set:
define( ‘WP_DEBUG’, true );
define( ‘WP_DEBUG_LOG’, true );
define( ‘WP_DEBUG_DISPLAY’, true);
so you can start seeing exactly what errors are occurring