Looking good! Post some pictures of the pcb in the doser when it comes in. I modded my 3s to do the same thing, but I'm curious if the 4s has the same design.
Temperature calibration isn't built in to reef-pi yet. We have issue 324 to add that feature, but it's not in progress yet.
It's common for a buck converter to buzz. They generally work by turning the power on and off very quickly, and basically averaging the output with an inductor (the black square with a circle in the middle). The coil in the inductor can vibrate and cause that noise. It will be more pronounced with more current.
That sucks that there is no temperature calibration. It seems like it would be very easy to implement. I would only want 1 point calibration so that I could add or subtract a certain amount. I have bought new buck converters as this one will drive me crazy lol. I will also post a pic of the doser board as I have no clue what I'm in for.