Android phones, apps and tablets

there's sufficient storage

I expect there's some lower limit to free storage beyond which you start having problems even running anything. If you used 1.2GB of that free 1.6GB, you'd have 5% free, which means no space for temp files. It's not Android being too stupid to do arithmetic, it's Android protecting you from yourself 🙂

Obviously my main recommendation would be not to use Zwift, but if you must then you probably have to do what it says and delete something else, or get a new device.