I agree, the parts of the air suspension are expensive and hard to find. There is not a huge market and there is not much information out.
But the MB air suspension in its engineering is superb and the rid in a classic MB air spring car is priceless.

So it is coming down to the repairs of these cars and this is the tricky part:
analysis and find the appropriate/reasonable way to repair the system so you can enjoy the ride.
I went the hard way (well I was trained at Maybach and MB in Germany and I am an engineer) and it took me 3 years to repair the system without buying anything from the major suppliers (except for the new rubber air springs).
However this is a hard way to get to the result. The result was great and the car stays up for 1/2 year or longer...and it works to specs.

I will publish an article about the air suspension soon based on my understanding of the MB air suspension. And I will open a web site for information and maybe offer help or even rebuilding level control valve.

For the sniffing of the leaks with refrigerant.... you should expose the system for only very short time with the refrigerant...don't leave the refrigerant in the system for long as it is not in harmony with the materials.
MB was even proposed this method in the beginning (older service manuals) but withdrew for certain reasons... exhausting green house gases in the atmosphere is illegal. Use soapy water that creates bubbles and find the leaks with this method is doable.
In my article I am touching on the analysis which can be difficult.
But if you understand the principles of system better you will find it easy to find and interpret the leaks and do the correct repairs.
A big portion of the difficulties is understanding the system. It is a complex system indeed.
Martin