As the a/c ages, cooling capacity will decrease. That makes this undercooling problem bigger with a very 1% moisture removing on the low end. The sad part of this story is that this undersized a/c is unable to maintain <50%RH during wet cool weather. Size for the extreme. Pulling the temp down after tstat set-up and being able to maintain desired temps with friends is big. If you live in a green grass climate, include supplemental dehumidification. Dehumidification is part of the package to deal will moisture from proper fresh air ventilation and occupants during cool wet weather. Everyone will be more satisfied and healthy. You overal operating cost will be lower and you home healthier. Regards TB
Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
Use +Merv 10 air filter. -Don't forget the "Golden Rule"
You should have called another HVAC contractor that would do a manual J and d on your home. Sounds like the contractor you are using doesn't know what he is doing or care. You shouldn't have to keep upgrading your system before he finds the right one.