If your SSC speed controller is the one with 6 separate LEDs it is doing what it should going to high speed if compressor runs continuously. this rules out compressor running to slow.
Yes, high pressure port can assist in dislodging something stuck in inlet of capillary tube. First you need to establish a performance base line of high and low side pressures across cap tube. After purging air out of servicing gauge set with refrigerant, connect both gauge hoses to service ports
. Now with a warm box start compressor in 10 minutes record
suction pressure, if below 6 psi add refrigerant to obtain 6 to 8 psi. This must be done in a time window under 20 minutes total running time. If you are unable to make adjustment in time window between 10 and 20 minutes turn off unit and let evaporator warm up and try again. When refrigerant volume is set as described run system 15 minutes more and record
both suction and high pressure and frost length towards compressor. This process means we have set refrigerant to match ambient air and seawater conditions so do not add or remove refrigerant any more. With thermostat set full cold record box temperature and high and low pressure along with frost line each hour for 4 hours or till suction pressure drops below 3 positive psi. Of course stop compressor if frost line reaches compressor.
After the above test, if performance is not satisfactory Yes, I would try to open capillary tube with Pressure and vacuum. Remove refrigerant pressure from suction low pressure side of system this will prevent loss very little oil
With little or no refrigerant in system Connect vacuum pump to high pressure service port and refrigerant bottle to low pressure service port. I would not use nitrogen as pressure above 100 psi can destroy Roll Bond aluminum evaporators.
I prefer standard thermometers and thermostats for boat refrigerators.