sometimes you can fix your issues with balancing the registers (dampers). Those closer to the fan/cooling source get a higher pressure, so they should be opened LESS than dampers farther from the fan.
Filters must be clean for HVAC to work as designed. Does it get better when you take the filter out temporarily? Maybe you have the wrong type/too-restrictive for your system.
Lower attic temp can help reduce system load.
Ceiling fan in the room, can reduce need for cooling, and they are cheap to operate.
You may even be able to select a higher speed termination on your existing fan motor. Note that speed might be too much flow for your ductwork.
Otherwise you could try smart vents and do time scheduling of rooms. Fun.