Three questions
1) How good the AC is when you are on a longer drive with constant speed? Is it good enough then?
2) Does the AC trip when car is parked? Does it turn off and then turn on?
3) When you spent 20K to change the whole system, did your mechanic install a new compressor or a used one?
I have a Cultus 2004 that has exact same issues with the A/C. My car AC Mechanic is known to us for many years and he said that unless I change the compressor with a brand new / genuine part, the AC performance will remain at par in the city.
My car AC does not trip on idling because compressor is weak and can't cool the air to 8 degree while at around 1200 RPM.
When it runs at 3K RPM for around 20 minutes, the cooling is excellent and AC trips as well.
Also keep in mind that peak temperature of the day is at about 4:00PM in the afternoon.