In general a platform-as-a-service solution runs the application code on top of a provider-supplied sandbox infrastructure which complements your app code so that together they implement the complete app functionality. But this puts specific requirements (often quite restrictive) in front of the application code, for example:
- a limited choice of (typically high-level) programming languages
- certain pieces of mandatory functionality in your app code (like health checks, etc)
- certain limitations of what the app can do (for example not allowing writes to the local filesystem)
So when looking for a platform-as-a-service solution you need to be looking from both the app perspective and the sandbox perspective.
Now your case brings additional requirements to the game:
- running a 3rd party app which you cannot modify, so you need to find a PaaS offering whose requirements are met with the application as-is, unmodified
- a Windows-based sandbox - rare, most providers use open-source infrastructure, I suspect primarily because of the licensing terms. Your highest chance of finding such offering would be on Azure, but not necessarily the only one
- AE licensing terms - how many AE instances can you run?
Due to the extra requirement on the sandbox a infrastructure-as-a-service solution would be, IMHO, a better fit for your case (you're actually using it presently, maybe just scaling it would be a much simpler solution for you?).
AFAIK the only offering (outside of Azure, with which I'm not familiar) that comes very close to an infrastructure-as-a-service but still is, in a way, a platform-as-a-service is Google App Engine Flexible Environment:
- meeting the app requirements might be possible by actually writing an app implementing a rendering service which uses AE under the hood, as a dependency
- it supports (docker-based) custom runtimes, you might be able to build one with AE/Windows (maybe how it's done for
ASP.NET apps would help)