Best practice, removing admin access from many websites
Maybe you could use Drush for these operations, such as change passwords for admins ? Also to write a script with several drush commands, which would run via SSH console.

Unfontunately, I am not expert in writing these scripts, but I think that you should look in this direction.

For Drupal 8 another solution Drupal Console has been developed.

P.S. Drush and Drupal Console hardly work on virtual hosting. If you decide to use it, you will have to host your site on VPS as least.