You can also have the stiffening plate avoid the mechanical interference with the strut fasteners and then not have to modify the length of the TE
I like this suggestion. So are you saying that the aluminum ring could be pierced to allow an open space for the top truss brackets to still contact the birch ring directly?
Correct. Realize that stiffening affects the entire component and does not have to be over the entire surface to work. Also, stiffness comes from the surface/shape of a geometric support, not the volume/weight of material.
The parts of the aluminum ring that would then be left going around the truss mounting brackets would not be very big. Might this be a place where deflection would still occur?
Unlikely but remotely possible. By the way, you should check the brackets as well as possibly cracked.
I would also inspect the bracket to see if it was not flat but convex - that would be an example of how you might get the appearance of flexure without any breaks, because the thrust bearing angle of the TE's might rotate/tilt because the bracket isn't fixed against the ring, allowing freedom of movement.
The general theme is to "stiffen" a structural member and see if the deflection amount changes - sometimes you don't need to stiffen everything.