Minimal Karabiner-Elements rule for selecting the input source between Japanese Eisu or Kana with command keys in the US keyboard layout.

left_command_pressed_alone_to_japanese_eisuu_and_right_command_pressed_alone_to_japanese_kana.json

{
    "description": "Change left_command/right_command pressed alone to japanese_eisuu/japanese_kana, respectively.",
    "manipulators": [
        {
            "from": {
                "key_code": "left_command",
                "modifiers": { "optional": ["any"] }
            },
            "to": [{ "key_code": "left_command" }],
            "to_if_alone": [{ "key_code": "japanese_eisuu" }],
            "type": "basic"
        },
        {
            "from": {
                "key_code": "right_command",
                "modifiers": { "optional": ["any"] }
            },
            "to": [{ "key_code": "right_command" }],
            "to_if_alone": [{ "key_code": "japanese_kana" }],
            "type": "basic"
        }
    ]
}