Hence why I thought instead of those bushes, which obviously must have been thought to be at least nominally ok, you replace the bushes with a block. You would at least have more longitudinal rigidity without having to change anything else. I'm not sure people are picturing what I mean so I'll give an example. PLEASE these are just numbers for an example.
Say you have a mount like yours. With 4x rubber bushes of dimensions 50mm diameter and 25mm thick. The distance between the bolts is 200mm. Instead of those bushes, you have a 'block' of rubber that is 250mm long, 50mm wide and 25mm deep.
That seems to me that it would have more rigidity than the 50mm bushes. I'm not saying it's the solution, but you also haven't said anything I didn't already know.
The blocks must surely be somewhat better than the bushes, which remember are considered ok else no one would fly with that setup. Again I don't know that it's the answer, but an improvement on something already being accepted, whilst not having to change anything else.