Steel framework damping suggestions?

[marbles]

Thanks for thoughts.
I concur. C is the priority, The X rail needs support in the middle and the A diamond provided that support. I think there is also some work to do on to gantry legs to get them absolutely square to the beam. Even a very slight parallelograming is creating a tension in the structure and i'll need to get rid of that. Watch this space :)

I would do them in the order C A D

With the A bracing it is better to go from the corners instead of midspan as these are stiffer.

The above would help with the visually detectable shake. The rest are more likely to improve tiny vibrations that effect surface finish so for what you need might not be necessary.