Yes, I know how to do it. Declare a variable which keeps track of the current "angle"/position in the sine wave. The "breathing" formula uses that angle, rather than calculating it from millis (). Each time round the loop, add a small amount to the angle. That small amount depends on the temp band. Lowest temp band = smallest additional angle, highest temp band = largest additional angle. Don't have to wait until zero before changing the wavelength, because there will be no sudden change to the angle, whatever angle that is at the time.
So its just a matter of calculating the additional angle for each temp band.
Paul