Isn't there method to avoid 'Update-dependence input timing'?

I found or tried something for detection of correct input time but…

  • fixed update input
    not in uniform interval. just be called at once before update

  • input system by fixed-update setting
    I was hoping for something different, but same with fixed update

  • OnGUI
    people said not working…

Is it right to give up at this point? Everything is update dependence after all.