The pressures look fine, ideal really. Afraid the R-4 just doesn't do enough work at idle to produce very cold air. Not sure whether that's due to the idle speed of the diesel, pulley sizing on the compressor, or displacement of the compressor.

At this point all you can do is make sure you're getting as much recirculation as possible. I'm not familiar with the 126; never owned one. But on the 123 the factory designed the a/c system to use 80% recirculated air and 20% fresh air. A common mod changes it to 100% recirculated air, which makes a noticeable difference in comfort on hot days. Not sure if anything similar applies to the 126.

Other than that, a good window tint helps a lot by reducing the sun load.