Try setting it in terminal now that you have added the newmode:
xrandr -s 1280x720
In this regard, if I were to make a recommendation for a Graphics Capable distro it would be Pop_OS
I do not for a variety of reasons; but please always remember that I am a novice. There is far, far more about what goes into a Linux system than I know or understand. You could be right and it is possible that setting a different resolution may resolve that. I would hedge bets that fractional scaling or Refresh Rate would have a far more pronounced effect, though.
Also- Please take a review of this article (and video) about Kdenlive choppy playback while editing:
https://www.netdip.com/2018/03/how-to-make-kdenlive-perform-better-and-stop-stuttering