AC not working

Never really heard low coolant make a sloshing noise inside the car but with water slooshing in the ducts yes. Also its impossible to tell when the ac is full by the pressure readings. Also the reason its not overheating is because the fans are always on with the ac on.

Check the high and low pressures if okay, test the pressure switch and AC relay-compressor could be locked-up

You may have over charged the system, the A/C uses a measured amount of refregrant if over charged the high pressure switch will shut the system down, in your case there should be 2.4Lbs or 1.1kgs

Bring it to a shop that deals with auto AC

If the magnetic clutch engages the compressor and still no cold air could be still low in freon or the HVAC unit

Take his advise

If it is low there is a leak, did they fix that if not you will be back again with the same problem.

Make sure your coolant is at proper level in both the radiator (engine cool) and in your overflow tank, and bleed your cooling system properly. Check your thermostat. If it opens at a lower temperature than it should, or if it's stuck open, you won't have a lot of heat coming out of the vent. If your thermostat is stuck open by the way, your temp gauge on your dashboard should read low (or lower than usual).
If none of these fix your problem, come back and we'll look at more possible problems (blend door, etc).

Its just has to be the heart of the AC system, sorry to hear that-Good luck

Does your car have separate temp controls for left and right side?