Okay, but if you have a 120Hz panel, as you said, you can just hold your 24Hz signal for 5 frames. And a minipc and TV can happily negotiate a 24Hz HDMI signal. Flicker fusion also has nothing to do with this unless you have like BFI or something.

If your source is 24Hz, it will never be 120Hz end-to-end. It's already 24Hz. You can just send that to the display. If your panel can't cleanly display that, it's not going to be able to with a built in app either. The app is also getting a 24Hz source video.