Load testing AWS AppStream fleet

There are industry standard tools you may want to explore for load testing. LoginVSI is commonly seen for VDI but I am not sure whether it is certified for AppStream. You may want to check with the vendor directly. The tool has wide range of capabilities for virtual desktops and apps.

Having said that, looks you only wanted to simulate login behaviour to validate scaling policies. Why not use common website load testing/automation tools such as Selenium or Jmeter? All you need to configure is to login to AppStream and click on the icon. This workflow is no different than any other website login testing.

I would strongly urge rethinking this strategy. Since this isn't Terminal Services, scaling out sessions doesn't have the same impact. Each user is provided a single instance and is not impacted by the existing user sessions. If you do end up testing on a larger scale, you will see each user should have approximately the same experience as the small testing you have already accomplished. And at the same time, you would introduce a lot of wasted cost.

Please review the maximum provisioning rate section of AppStream 2.0 Best Practices to get a better understanding. You will also want to ensure you have followed other sections in the guide, ensuring the architecture includes multi AZs to scale horizontally and more efficiently.