The A/C cooling and engine shaking could be two different problems. Does the idle speed go too low when the A/C is on? If so, it could be the base idle is set too low or perhaps the IACV-AAC valve is faulty. A bad engine mount might be another possibility.
As far as the cooling issue, you should really not add refrigerant if you don't know how much is in the system or can view (and understand) the high and low system pressures with a set of gauges. Overcharging can cause issues just like undercharging. If the proper amount of refrigerant is in the system and the compressor clutch is engaging when the system is on, I would make sure the (1) engine coolant is not getting to hot when the system is not cooling as it should, (2) the electric cooling fan is operating and operating at the proper speed when the A/C is "on," and (3) there is no restriction or blockage of airflow through the A/C condenser, like leaves built up in front of the condenser or between the condenser and radiator cores...or bent condenser fins or debris clogging the fins of the core.
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