Drush CC all returning "No Drupal site found, only 'drush' cache was cleared"
There are a few things you can do to help debug your drush. Here is what you can try:

Debug output

Use drush cc all -vvv

This will provide you with some valuable information in terms of what Drush is doing in the background and maybe show you the actual error message.

Check your settings.php

Depending on your setup you may be passing in environment variables to your settings.php file for a database configuration for instance. Or perhaps to provide memcache. Drush unfortunately does not pick these up as it's bypassing apache. Therefore I would give it a test run without environment variables to see if that eliminates the issue or help narrow it down. For debug purposes it may be worth just using a clean copy from the example.settings.php and just modifying the database array to test this.

Check the mysql client is available

This is one that tripped me up a while ago and the drush -vvv will point you in the right direction. If it can read the settings.php, which it appears it seems to be able to as it gets the db settings, then perhaps it can't connect to your mysql database. One of the issues I had was that mysql client command was not available. E.g. try calling 'mysql' and see if that works. After fixing that it worked. In my Mamp setup it was just a matter of ensuring the correct path to the executable was in my shell $PATH.

I hope this helps.