After much research, head scratching and changes of plans… I *think* I have a workable solution for my sim intercom systems.
I had gone down the track of hardware intercom solutions, even as far as using headset distribution amps and mic mixers, but after quite a lot of research, and a suggestion from an internet post I have found a really simple solution:
Basically it is a software mixer, which enables both internal intercom and a connection to and from either TeamTalk (which I use between all my sims) or Squawkbox when flying on VATSIM. Only required hardware are USB sound cards for each headset, plus any impedance matching circuitry for the individual headset. So.. ALiexpress order placed for a bunch of cheap sound cards… and then to set it all up!