Kanata (https://github.com/jtroo/kanata) config to activate mod keys and enable fast scrolling when mouse buttons are being held for more than 300ms.

mouse_mods_kanata.kbd

(defvar
    tap-time 200
    fast-scroll 1200
)

(defsrc
    mfwd
    mbck
    mmid
    mwu
    mwd
)

(defvirtualkeys
    sftx lsft
    ctlx lctl
)

(defalias
    psf (on-press press-virtualkey sftx)
    rsf (on-release release-virtualkey sftx)
    tsf (multi @psf @rsf)

    pct (on-press press-virtualkey ctlx)
    rct (on-release release-virtualkey ctlx)
    tct (multi @pct @rct)
)


(deflayer base
    (tap-hold $tap-time $tap-time mfwd @tct)
    (tap-hold $tap-time $tap-time mbck @tsf)
    (tap-hold $tap-time $tap-time mmid lalt)
    (switch
        ((and lsft lctl)) (macro @rsf @rct 10 (mwheel-up 50 $fast-scroll) 10 @psf @pct) break
        () (mwheel-up 50 120) break
    )
    (switch
        ((and lsft lctl)) (macro @rsf @rct 10 (mwheel-down 50 $fast-scroll) 10 @psf @pct) break
        () (mwheel-down 50 120) break
    )
)