Reluctantly I have disabled Amazon Polly in P2A.  It's voices are amazing, but the implementation on the desktop is buggy.

In P2A Polly would work for a while, but then it would start acting up. The worst was that it seemed to cause P2A to lock up during flights. If I paused a flight in MSFS by hitting ESC, P2A would free up and start working again, but as soon as I reentered the flight, P2A would freeze up again. Apparently something in Polly caused this behavior, because it didn't happen before I started using Polly, and as soon as I removed the Polly voices from P2A, it stopped happening.

Even Polly Player would expose errors in the implementation of the system. For example, this is the error I would get trying to use several of the voices.

I'm disappointed in this. Polly voices are among the best you can find now. Unfortunately, until Amazon fixes these issues, they are really unusable in P2A.

While aspects of the Polly implementation on Windows are poor, if you find the working voices within Polly Player (this may vary from those listed in the API, as your above example demonstrates) and make sure to only assign THOSE voices within P2A, then the system is quite useable - once you get past the initial pain of (potentially) restarting P2A several times before it works, that is!

With no action on the Polly GitHub repo in the last 10 months, and no official feedback on the Issues page, it would seem that Amazon has lost interest in this project - for now, at least. Hopefully, they will pick it up again soon but I am not holding my breath.