Depending on the load you either have an undersized evap or over sized c/u. That is a 3000 btu evap with a 8000 btu (w/30° evap @ 90° ambient) c/u that was never designed to operate in the temperature range for wine storage.
You want your system to have about a 10° TD @ 45° SST.
Currently your coil is way to cold and if it isn't creating a dry environment it's only because of the short run times due to over sizing.
What is the size of the room and what kind of insulation is there? What size is the TXV?
There is absolutely no reason for a defrost cycle.
Perhaps an EPR may solve your issues but if the client has a valuable collection I'd rip it out and start from scratch.
A wifi enabled tstat should be an easy up sell on this job.
My WAG is the system has always had high head pressure and that's why the first c/u only made 5 years.