It should be fine as ffmpeg does not run in the background so if you don’t use the features it will not be running at all and the binding will still work the way it does for you now. It is mainly a cpu using program not a ram hog and as mentioned if the cameras feed is h264 which is every cameras default the cpu load is next to nothing as it does not Re encode for HLS which is what u want for casting.
HLS uses the storage device to hold around 10 seconds worth of video so with slow sd cards it is an issue, which tmpfs solves this.
You really should not be scared to give it a go. This shows you do not trust your backup to work. Copy the sd card to a second card and try it out on the copy then any issues just pop your unchanged card back in, easy, no risk and no need to worry.