All of this looks very complicated. What advantages does this have over other immutable systems, like fedora silverblue or opensuse aeon?

Imo at this point you’re better off just nuking everything (after you backed up your important files) and reinstalling with the following process:

• Install windows, with the linux drive removed
• Plug linux drive back and install ubuntu on it (make sure you select the right drive in the installer) If you do this ubuntu should automatically detect windows. If it doesn’t you should still be able to boot into either one of them using your device’s bios.