I found that my compressor was low on r134 (negative pressure). After searching the internet for the correct pressure, I found all kinds of pressures, ranging from 55psi (I finally decided was for automotive or a/c, even though claimed to be for a refrigerator) to a more reasonable 3 to 5 psi. As my compressor was made by Whirlpool, one pundit said it should be on the lower number, ie 3psi.
I have used this number, but my technical information manual, says 0psi. Experimentation seems to indicate 0psi doesn't cool very good, so I am using 3psi, which gives me a little frost on the suction line (can't see any frost on the evaporator).

My unit now cools to freezer 0 degrees and refrigerator 40 degrees, but requires the control to be turned up (cooler) higher than in the past. Could the compressor also be weak?

Even 2psi frosts the tube to the compressor. I'll try lowering to 1psi.

If you have heavy frost in the freezer coils, the system will run too cold and the R134A vs temp chart will indicate a lower pressure, best to clean coils first.