Kev, why make things more complicated than they have to be? Cleaning curtains in-situ is all about removing the dust and insects... dry extraction.
A good turbo vac, detailing with cylinder vac & brush attachment followed by a very light mist of m-power or whatever you like, wiped off with a terry towel, bob's your uncle.
If curtains look dirty and the client wants them to look clean, why take on the stress... get them down, in to a dry cleaner and transfer the responsibility.
I dont stress Jim, and like I say weve been doing them for yonks (as you described) - its just that I fancied getting away from solvents - and maybe looking a tad more professional at the same time if thats possible, especially in front of commercial clients.
I wouldnt dream of using a normal steamer for instance (as per youtube 'how to steam clean curtains') as there is NO actual cleaning involved - it just 'looks' like they are being cleaned (they may be sanitized to some extent, but thats it). If I had one that extracted at least I could be honest and say that I am extracting what soil I can from the material - and in a healthy, safe way.
Following on from this though, generally speaking so long as they have seen someone 'cleaning' them they are more than happy! so maybe I should buy a cheap steamer and 'pretend'
The thing is with curtains they never come out like new and we stress that big time, although some pile curtains if grubby do get transformed.
Cheers
Kev