Can you hear the compressor clutch relay pull in? Check by turning ignition ON and turning ac ON with hood open. Should hear a distinct click.

Relay located (on my '83) under a 5" square cover on the driver side inner fender. It is one of those general purpose "ice cube" relays. I found that mine worked loose under vibration and jolting. Pressing it in and securing with a tywrap fixed my problem.

BTW, I too converted to R134 on my 1983 300CD. Works great with outlet temps as low as 29.8 deg F on a cool morning before leaving the garage. My conversion was complete as yours is except for the condenser. That too will eventually be changed to a modern parallel flow unit.

I don't recall whether or not you changed the hoses. My ac maven said they should be changed because of the difference in molecule size between R12 and R134 and subsequent slow osmosis leaks.