why no 24hz support?

Support from nvidia is more or less non-existing.

This is the custom /storage/.config/xorg.conf that I used to use with a GT520M: https://chewitt.libreelec.tv/xorg.chewitt

The main point is "NoEdidModes" to ignore what the TV reports over HDMI and then hardcode the modelines. The original reason for doing this is to increase the accuracy of the fractional rates (23.976, 29.97, 59.94) as Xorg default calculates to 2x decimal places and this results in occasional skipping, and increasing this to 4x decimal places with fractional rates improves modeline accuracy. It will not eliminate skipping, but it will happen less frequently - to a level where it's rarely noticeable.

The modelines in that file are based on what I saw reported on that specific TV, so no guarantees they will work on your TV, but you can crib the format and experiment with likely values that you see in the systemd journal with Xorg started in debug mode.