Why not make a subspace strategic missile armed with a meson warhead? It would be fairly simple, but extremely effective. It would basically be a heavy armor shell surrounding a single use subspace engine, sublight drive, guidance system and meson warhead. The missile could be launched from a dedicated destroyer, or a distant installation, jump to subspace, jump out in proximity to the target and then dive into the it, using a heavy application of ablative armor and speed (burning out it's single use engine in the process). Could even take it a step further and install multiple independantly targetable meson warheads. If they could be produced in large numbers it could allow the GTVA to counter the huge numerical superiority of the Shivans. (Would make a really boring campaign though...)