More air-struts alternatives (besides Arnott and Bilstein)
Miessler: https://www.miessler-automotive.com/Suspension_636
Aerosus: https://aerosus.com/jaguar/xj/x358.html
...both make it seem they're German, but especially with Aerosus I have my doubts (I am not sure I would care though)

Also my understanding is that with Arnott you don't get the CATS-integration.

 

(At least) one of the Chinese makers - ISTR it was Suncore? - DID make units with the modulation solenoid valving. "For a period of time". Best I can tell, the demand for them has tapered-off to where it is no longer worth even running a batch once a year or so. Note that OEM Bilsteins may be out of stock part of the year as well.

Presently, no, but you did at one time. Back in the early daze when Arnott started-out by putting new airbags into rebuildable shocks, retaining the OEM solenoid.

The solenoid valves themselves are made by at least two firms - A Japanese HQ'ed multinational one with German production facilities is probably the supplier to Bilstein. Another firm in Belgium or the Nederlands might be Koni's source?

They are expensive partly because dynamically modulated dampers don't provide enough value-add over fixed-rate dampers to hold high market share so as to drive the cost down on high volume competition.

As an example for contrast, look at how cheap German-Made Bosch DFI injectors and German-Made HPFP units have become.