As some others have said, the problem is probably that windows still has the drive locked. When windows "shuts down", it actually is only closing your programs and going into hibernation. This leaves the drive in a read only state, which will prevent you from being able to resize the partition.
To do a full shutdown, you can hold shift while pressing the shutdown button on the start menu. Alternatively run shutdown /s /f /t 0
in a administrative command prompt.
https://github.com/ryanrudolfoba/SteamOS-Waydroid-Installer
Not disagreeing with your point, but at least for waydroid there's a specific SteamOS installer available.