AutoHotKey v2 script to activate mod keys and enable fast scrolling when mouse buttons are being held for more than 300ms.

mouse_mods.ahk

; Hold time threshold in seconds (0.3 = 300 ms)
HoldTime := 0.3

; Multiplier for faster scrolling
ScrollSpeed := 10

; Timers start disabled
SetTimer(CheckHoldCtrl, 0)
SetTimer(CheckHoldShift, 0)
SetTimer(CheckHoldAlt, 0)


; --------------------------------------------------
; Back button (XButton1) -> Shift key
; --------------------------------------------------

XButton1::
{
    global HoldTime

    if KeyWait("XButton1", "T" HoldTime) {
        ; Quick press
        Send("{XButton1}")
    } else {
        ; Held
        Send("{Shift Down}")
        SetTimer(CheckHoldShift, 10)
    }
}

CheckHoldShift()
{
    if !GetKeyState("XButton1", "P") {
        if GetKeyState("Shift", "P") {
            Send("{Shift Up}")
        }

        SetTimer(CheckHoldShift, 0)
    }
}


; --------------------------------------------------
; Forward button (XButton2) -> Ctrl key
; --------------------------------------------------

XButton2::
{
    global HoldTime

    if KeyWait("XButton2", "T" HoldTime) {
        ; Quick press
        Send("{XButton2}")
    } else {
        ; Held
        Send("{Ctrl Down}")
        SetTimer(CheckHoldCtrl, 10)
    }
}

CheckHoldCtrl()
{
    if !GetKeyState("XButton2", "P") {
        if GetKeyState("Ctrl", "P") {
            Send("{Ctrl Up}")
        }

        SetTimer(CheckHoldCtrl, 0)
    }
}


; --------------------------------------------------
; Middle mouse button (MButton) -> Alt key
; --------------------------------------------------

MButton::
{
    global HoldTime

    if KeyWait("MButton", "T" HoldTime) {
        ; Quick press
        Send("{MButton}")
    } else {
        ; Held
        Send("{Alt Down}")
        SetTimer(CheckHoldAlt, 10)
    }
}

CheckHoldAlt()
{
    if !GetKeyState("MButton", "P") {
        if GetKeyState("Alt", "P") {
            Send("{Alt Up}")
        }

        SetTimer(CheckHoldAlt, 0)
    }
}


; --------------------------------------------------
; Faster scrolling when both side buttons are held
; --------------------------------------------------

*WheelUp::
{
    global ScrollSpeed

    if GetKeyState("XButton1", "P") && GetKeyState("XButton2", "P") {

        if GetKeyState("Ctrl", "P") {
            Send("{Ctrl Up}")
        }

        if GetKeyState("Shift", "P") {
            Send("{Shift Up}")
        }

        Send("{WheelUp " ScrollSpeed "}")
    } else {
        Send("{WheelUp}")
    }
}

*WheelDown::
{
    global ScrollSpeed

    if GetKeyState("XButton1", "P") && GetKeyState("XButton2", "P") {

        if GetKeyState("Ctrl", "P") {
            Send("{Ctrl Up}")
        }

        if GetKeyState("Shift", "P") {
            Send("{Shift Up}")
        }

        Send("{WheelDown " ScrollSpeed "}")
    } else {
        Send("{WheelDown}")
    }
}