First, one quick correction: the upper limit of nose breathing would be the top of Zone 2 rather than Zone 1.
Second, if your breathing doesn’t feel noisy and labored, then I think it’s safe to say that you’re below your aerobic threshold. (The top of Zone 2 is marked by the aerobic threshold.) In that case, you could continue at that intensity or increase your pace until your breathing becomes noisy and labored.
Third, it’s not necessary to train right at your aerobic threshold all of the time. The “noisy and labored” quality is just a good benchmark to feel when you’re there.
Fourth, it is possible to nose breathe above your aerobic threshold, but usually, it happens if you haven’t warmed up slowly enough.
I hope that helps.