HVAC tripping breaker - things to check before replacing?
The condition you describe is usually an indication that the compressor is unable to provide sufficient start torque to turn without drawing excessive amperage. This is frequently due to a weak run capacitor on the compressor.  Less frequently this can be caused by a contactor in poor condition or an old compressor where the bearings have worn to the point that the unit requires higher starting torque than normal.  Given what you have already eliminated, the cure to this problem is very likely to be one of, or a combination of, the following:


The run capacitor for the compressor is weak (most likely).  This cannot be determined by visual inspection.   If not already checked with a meter, then disconnect the capacitor and measure it with a capacitance meter (the main disconnect must be OFF).  If less than 90% of the labeled value, then it must be replaced.  Multi-meters with a capacitance scale are less than $20 from Amazon and others.
The contacts on the compressor contactor are badly pitted.  This can cause a high resistance connection in the contactor.  Visually inspect the compressor contactor for any signs of heat damage.  If possible (with the main power off), remove the arc-shield and inspect the contacts for pitting.  If there are many deep pits (greater than 1/32"), then the contactor must be replaced.
The compressor is a bit worn.  To provide the increased starting torque necessary, install a "hard-start" kit for the correct horsepower unit that you have (roughly, your compressor motor will be 1 HP for each 12000 BTU cooling capacity).  I prefer the 3-wire type of hard-start kit because it is more stout, although these are more difficult to find online than the 2-wire type.  The 3-wire kits are readily available at A/C supply houses and perhaps your next technician can help you with this.  If this is the problem then your unit may be on its way out; however, compressors can run many additional years with the correct hard start kit installed.
Install a 3-5 min "delay on make" on the compressor contactor coil (a $10 part).  If you install a 2-wire hard start kit, it is mandatory that you also have a 5 minute delay installed.  (Note that if you cycle the breaker for the compressor while unit is running, the delay on make will not be triggered and you may still trip the breaker with this action).