It sounds like you've done everything else, a larger fan might make the "stop and go" performance closer to the "highway" performance. The compressor will still be turning more slowly, but the heat transfer should be better. You need everything you can get in that humidity. I don't know if the old fans can slow down, mine appears to be operating at a very healthy speed (I can hear it from inside the car when it starts). It would be interesting to see data if you decide to try a larger fan.

BTW, I'm assuming your evaporator drain isn't clogged.

From looking at my W123, I don't know how easy it would be to install a larger fan with the braces in front of the condenser. It appears that the braces would have to be "redesigned" to make some extra room.

Fortunately, I spend very little time in traffic. My comments were based on mostly highway driving.