I am not a pro.
To check the freon charge correctly you need a set of gauges.
If the unit only has frost/ice where the freon enters the evaporator coils and the compressor is running continuously then there is a sealed system problem. Usually it is low freon but could also be a compressor problem or a partially plugged orifice.
The system should be evacuated and then re-filled and special equipment is required also most places require that the tech be certified to do this work.
Gets expensive with no guarantee of a long term fix.
Putting hundreds of dollars into a unit to get perhaps an extra year out of it just does not make sense to me. You may get a couple months or a few years or even longer, I cannot say as it depends what is wrong with it but just to diagnose it will probably set you back a couple hundred.