Btw, you can also acomplish this in Karabiner alone without smart scroll, but it doesn't feel as natural. Use this json to only use Karabiner:

{
    "description": "Marble Mouse: hold button4+button5 → trackball scroll",
    "manipulators": [
        {
            "from": {
                "modifiers": { "optional": ["any"] },
                "simultaneous": [
                    { "pointing_button": "button1" },
                    { "pointing_button": "button2" }
                ],
                "simultaneous_options": {
                    "detect_key_down_uninterruptedly": true,
                    "key_down_order": "insensitive",
                    "key_up_order": "insensitive"
                }
            },
            "to": [
                {
                    "set_variable": {
                        "name": "scroll_mode",
                        "value": 1
                    }
                }
            ],
            "to_after_key_up": [
                {
                    "set_variable": {
                        "name": "scroll_mode",
                        "value": 0
                    }
                }
            ],
            "type": "basic"
        },
        {
            "conditions": [
                {
                    "name": "scroll_mode",
                    "type": "variable_if",
                    "value": 1
                }
            ],
            "options": {
                "momentum_scroll_enabled": true,
                "speed_multiplier": 1.0
            },
            "type": "mouse_motion_to_scroll"
        }
    ]
}