That seems backwards thinking to me. Either a synod should be infallible or not. Either it is protected by the Holy Spirit or it is not. If it should be and the teachings contradict prior teaching, then we have a serious problem on our hands. In my mind, that would just be more of the same ala Vatican II.
I think that's because you believe synods, by definition, are part of the ordinary magisterium. I do not, especially using the definition of the ordinary magisterium you posted: Ordinary Magisterium is the perennial teaching of the Pope and the Bishops in union with him around the world. I believe they are part of the ordinary magisterium when they teach something that is "the perennial teaching of the Pope and the Bishops in union with him."
I believe synods can deviate from that, because they do not consist of all the bishops. No one would argue, for example, that the Synods of Baltimore were infallible by nature. The fact that they did not teach anything in conflict with the ordinary magisterium does not mean that they could not have done so. It only means that we're fortunate that they did not.
I could be wrong, and I will look into it further. But that's why I'm concerned about what's going to come out of the upcoming synods. And again... even if they are infallible, that doesn't mean that every disciplinary decision that comes out of them is going to be a good idea. That's why I pray every day that these upcoming synods will be faithful, clear, and effective in bringing souls to Christ.