I know this is a bit of an old post, but in case it helps anyone……
I’ve got a 150 liter chest freezer that I use for fermentation chamber. When I started using it I did start to see the odd bit of mould here and there. Using an inkbird for controlling the cooling and an old hambleton bard heat pad for heating. So I bought a 24 volt 80mm cooling fan and a 12v power supply so it runs slower. Keeping the air moving seems to stop the mould dead and have not seen any since moving to this setup about 6 months ago. Has the added benefit of keeping the temperature set point to +/- 0.3 degrees according to the data logger. Previously it would swing up and down round the set point by 3 degrees as heating and cooling were fighting each other no matter what deadband I used on the heating/cooling differential. Also put the same setup in my serving keezer as noticed the difference in temp between the top and bottom of the keezer could by as much as 6 degrees. Moving the temp probe halfway helped, but again, now only 0.3 degrees difference.