I do not think that option should be needed. You can see the command line that Shotcut generates for proxy in View > Application Log. It includes -g 1 -bf 0 for no B frames. It is working for me on RTX 3060. There is no easy way to modify this command line if you are not a regular Shotcut developer. I followed your link and its links to read more about this. It looks like it might be an issue with older cards. It is ridiculous that this option needs to be added; however, I can add it for the next release (but not the beta that was already prepared this morning). You can get it from tomorrow’s daily builds in GitHub Actions.