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I'd want to get a mirror in there and have a good look at the evap coils. Squeaky clean or totally clogged? I bet clogged. With very restricted air flow the system works hard but exchanges little heat and often freezes up. A thorough coil cleaning and recharge might get you in good shape for $500ish. Worth a look.

Another option is ductless mini-split systems. They are super efficient and very quiet. Traditional HVAC firms generally don't like them so you will need a specialist installer but you may be able to cover your whole house for $5k with a 18 seer system. Cool only the rooms you want to cool or cool the whole house with modern digital zone controls.