In the end, I decided to remove IsolatedStorage and utilize preprocessor directives, allowing me to easily work with files and directories in special folders through System.IO.File and System.IO.Directory.
The solution is incredibly simple, yet I couldn’t find it anywhere on the internet.
As you can see it works with Android, UWP and also OpenGL.
It doesn’t even require storage permissions.
#if __ANDROID__
this.Folder = GetFolderPath(SpecialFolder.ApplicationData);
#elif NETFX_CORE
this.Folder = Windows.Storage.ApplicationData.Current.RoamingFolder.Path;
#else
this.Folder = Path.Combine(GetFolderPath(SpecialFolder.ApplicationData), "NazdarTheGame");
#endif
(NETFX_CORE = UWP)