Hot take: Don't FF for toxic teammates. Make them play out the loss.
If your teammate is being toxic for 0 reason and not co operating when being asked to, in my opinion you should not FFT, but instead stay half afk and stay in the game till the end of the clock.
Sure you might lose MMR and waste time, but people can learn not to be toxic and they might rage enough to the point where they hop off the game.
That's just my take on it, if you are toxic, I will not be playing and wasting our time.