5 to 5 1/2 * low side.

175 to 193.

I agree with your suspicion; just low on R134a. The compressor may be losing some efficiency though. Try to get the low side up to 40 psi and see what happens. Compressor efficiency is not directly proportional to pressure. If you fill the system it might start working like gang-buster.

How quickly does system pressure equalize after shut down? This will tell you if the accumulator needs to be replaced.