I don't know that it really matters where the freezer's control is set - your additional temperature controller will determine when (how much) the freezer cycles to stay cold. On my similar setup - a freezer used for fermentation chamber - I have the freezer's controller set fairly low and it does fine, but again, if it were set lower it wouldn't matter because it shuts off when my temp controller tells it to. I question how accurate your reading would be with the probe taped to the side of your fermenter (bucket, etc). That will not reliably give you the actual temperature within your fermenting wort, which can run higher due to exothermic reaction in the yeast activity. A thermowell is more ideal for this, but without one you would do just as well to have your probe
in a decent sized thermal mass that changes temperature more slowly than the ambient air in the freezer. A beer bottle would approach this but before I got a thermowell I used a gallon jug filled with water to hang the probe in - more mass, slower temp change = the freezer holding more reliably within a set temperature versus rapid cycling on and off if the air inside the freezer warms. I feel this is no worse and likely better than a probe taped to the fermenter, even if insulated.