Modern AC units are completely sealed, no Sharader valves are on units made for some time now as a general rule. Bottom line, if you are low on refrigerant then you have a leak...that ain’t good. To prove that you have a leak is PIA. Rather than evacuating the unit, a sniffer might tell the tale. A tap valve can check as an isolation tool, but there will be a valve needed to be soldered on.
That being said, concentrate on the cheap and easy first, which is blocked air flow. Check the very thin filter, it is usually right against the intake side of rhe coils and hard to see slides up. Pull it and wash it. Home Depot sells coil cleaner in a rattle can for less than $6 as I recall. With filter out and AC running, spray the foam on the coils. Condensation will wash the foam to the pan and clean the coils, replace the filter. See if that works, then move on to duc’s as mentioned in previous post.
If this does not work, well good luck. You will have to make the decision to attempt to repair or replace. The units are almost disposable. Considering cost of hired help, many, if not most of the time it is better to bite the bullet and get a new warranted, highly efficient unit. Sure wish I had done that with my cursed Westerbeke Genset.