What about giving everything 1 health? As long as the planet is big enough, there won't be any commander rushes. Walls do little but block a single shot (which means quite a bit, considering that's one extra enemy unit that won't kill your defensive structures), and normal structures block two (blocking the shot and wreckage also blocking the shot) Alternatively, everything has normal health, but wreckage has precisely two billion health. Excellent for metal farming, but requires a ton of energy to maintain, which metal extractors could provide without. It'll also create a junkyard-like terrain, where there are a ton of broken units scattered around the battlefield, blocking paths and creating cover. Or both!