• The package managers and official repos for most distros would be better thought of as lego blocks to build an OS from - they have no concept of OS and application separation, and splitting installation of an OS across multiple physical drives doesn’t really make sense.

    Application focused distribution methods with a clear separation from the OS like Flatpak or AppImage do support this.

    AppImage - drag the .appimage wherever you want it.
    Flatpak - supports system and per user installs (under home directory) by default. Additional installation directories can be configured, but I’m not sure if any of the GUIs expose this feature - so likely doesn’t currently pass your bar of not needing to use the command line at all.