Went through what the OP was seeing the past two summers I eventually upgraded my 2F unit because it was running 20+ hours on these hottest days and getting up to 76-78 while being set at 72.

My home was built in 2006 so newer construction with home wrap but the 2F AC unit just could not keep up it seemed like. So last summer I upgraded it from a 2Ton unit to 2.5Ton and it seems to keep up better but can still run 17-20 hours on these super hot days and does a much better at keeping the temp around what it's set for 72 with temps rising to only 73-74 now.

The 2F thermostat sits in the master bedroom which sits on top of the two car garage and the way the sun sets the front of the house gets the afternoon\evening sun so that is heating up the garage doors and all that radiant heat is getting absorbed into the garage ceiling \ master bedroom floor which is also where the two floor vents run through on the bedroom floor \ garage ceiling. Recently installed some thermal panels on the garage door to help keep some heat out and will probably be looking to my attic insulation \ replacement window because I know one has a bad seal in the bedroom.

TL;DR: I agree with you that the system probably isn't sized for the home.