So, I made a thing... The AWS SDK for PHP is very basic when it comes to types and needing tons of asserts just to stop PHPStan from complaining annoys me.
This extension knows exactly what type each field of each AWS SDK call is and thus your PHPStan stops freaking out about everything being mixed.
https://packagist.org/packages/rikudou/aws-sdk-phpstan
Note that I literally finished it about an hour ago, so I'm not tagging a release version until I sleep on it.