Idle thought: I don't think I've ever seen one of these TV emulator things implement the situation where the vertical oscillator was slightly wrong and you get the picture slowly looping up the screen.
I actually posted ntsc-rs as it came up in my research - I'm also looking for something like what you're describing..!
I was also looking into https://codeberg.org/fsphil/hacktv which generates a variety of different analog tv signals (meant to be broadcast using HackRF) - but yes, I want the opposite - an analog-receiver-emulator...? And one that would be "ok" with incorrect signals // fail like an analog TV would... :-)
This one does. You can configure the noise injected into the signal and when it gets too much, it loses sync and the picture starts rolling. It's actually a software NTSC modulator/demodulator, not just an effect to simulate it.
https://github.com/LMP88959/NTSC-CRT