I've read this and was told by an HVAC friend to just keep the fan "on" instead of "auto." Any thoughts?
I would suggest installing an energy efficient HVAC blower motor before doing this. the permanent split capacitor 'PSC' motor in older units are horribly inefficient, it will cost roughly 8 cents an hour to run all the time, or $56/month. Contrasted to the ~$150 MARS 10860 1/2hp azrure EC motor (electronically commuted motor) will reduce the power usage from 900VA/650watts to 240VA/240W (at full speed) and it has an 'always on' lowest speed that circulates the air constantly while sipping only 30 watts ($3/month). This will help the OP with undersized runs. as I understand it, as of 7/2019 all furnaces have to have ECM motors to be energy star.
on the idea about closing all the vents first floor, i think that will cut down the total airflow in the HVAC unit too much its better to leave it all wide open and solve the issue another way. Also putting a digital thermometer in the vent and verifying that the cool air is 15F lower than the air return temperature and also making sure the AC is running for between 10 and 15 minutes at a time are rules of thumb for AC system health. some thermostats also allow setting how many times the system will cycle per hour, i have mine set to 2 cycles so that it runs longer when it does come on.
so much good advice here, recapping: R50 attic insulation, penetration sealing in the attic, black out curtains, all these are under $1000 total for DIY. then avoiding portable AC units but embracing inverter-based mini-splits or window units. And I would add dollar store automotive sun reflectors in East, south and west facing windows, but especially west facing windows.
I had the same issues >12F difference between the upstairs and downstairs in the summer and a $200+/month bill to show for it, the stated upgrades have reduced this to 2F, and a power bill that is MUCH less. I also check the weather and if its going to be cooler than about 74F and not too humid or raining, then I open the windows at 10pm and close them at 9am with window fans to pull in the cool night air, why pay for what nature sometimes gives for free.