I played a bit with your implementation of IsolatedStorage, and these two lines are not working:
And I can't seem to find the counterpart of Windows.Storage in the PSM SDK documentation (https://psm.playstation.net/static/general/all/psm_sdk/2/doc/en/).
Also, System.IO.Stream.CopyToAsync(System.IO.Stream, int, System.Threading.CancellationToken) doesn't seem to be available on PSM...
I wonder if there are any MG devs who are familiar with PSM and are willing to help implement this helper assembly? >_<