Part of your problem is the lack of insulation on that attic level. Reducing the heat gain will increase your cooling. Also, replace that octopus of poor workmanship with a well-insulated trunk duct and connect the flex to it. Then insulate the attic floor and cover the flex with more insulation batts. Each of those individualt flex ducts has a tremendous surface to absorb heat. A single trunk with 4-inches of insulation around it would reduce your cooling problem by reducing the duct surface gain. The cooling air is being heated by the 140+ attic.
I can see from the way the system was installed that it was done by a well-meaning, but inexperienced installer. He had the right idea, but not enough knowledge of heat transfer through ducts to preven the rise during the day.
A nice brand-new high SEER unit would cost a bundle and make the installer very happy; it would also increase your operating costs. As the exisitng unit is working, keep it, improve the insulation value of both attic and ducts. If that is not enough later, just install a 1-1/2 ton ductless split unit in the main living area and you will supplement the most inexpensive cooling with a reasonable installation cost. For now, a $200 window unit would make up the difference.