We go through this same routine on our 1998 car. I just have the system evacuated and refilled with the proper amount, I think it's 2.2 lbs. It does a reasonable job cooling thru the summer.
Yes, there is a minute leak somewhere and they've done the dye tracer but still can't find the problem indicating it's located somewhere that's going to be $$$$ to repair.
It's nice these cars have such large sunroofs
(the stop leak stuff makes the guys at the repair shop really mad, and if you confess to using it they won't hook up to the car
)