We have engine driven R134a water cooled system, 20 years old.
Has run well for years, since last work about ten years ago.
The freezer cools down fine, but the fridge is much slower to pull down than it used to be.
There is a solenoid valve that cuts off refrigerant to the fridge when it is at temperature. It used to close after a while running, typically about half an hour, but that does not act any more since the fridge is so slow to cool
The only components unique to the fridge are the expansion valve and cold plate, suggesting the valve may be an issue. How to check it? (I am an amateur)
I tried warming up the expansion valve in the fridge to melt out any ice, but this does not seem to help

I added about 200 g R134 (6 oz or so) and the compressor got a little noisy so bled a little off.
How to tell if I have sufficient refrigerant in the system? The sight glass has always bubbled.

I am wondering whether there is air in the system. Would it make sense to get a technician to vacuum it out and refill?

we are in Spain, and have not yet found a good service. I suspect we will be better in the Canaries where it is warmer, so more refrigeration systems.

We have a gauge set and plenty tools, but are amateurs at refrigeration, relying on Nigel Calder's book a lot.

The system is non standard, put together by a Fleming in the 1990's. He did boat and small commercial systems. We have no data on the correct volume of refrigerant for the system.

Ideas, and direction to good reading would be helpful