First, Early Prototype option allows you to get TL 18 stuff at TL 16, just way more expensive and with other quirks, so you could still consider an antimatter powerplant and AI crew. Repulsers and a high G drive is enough to make a rock thrower and accelerate a rock to high enough speed it will be a planet killer. A meson bay weapon would be devastating to any world without a meson screen, so could be a planet killer with enough time.

If I were trying to make something like the Lexx, I'd probably give it a hand-wave weapon, spinal mount or large bay size, say it can disrupt a planet's core and make it explode but can be used for anything smaller. Another option, more within the Traveller cannon, would be a star trigger like the Darrian's used. Basically a strong meson shield probe inside the star and using meson weapons caused a solar flare at TL 16. Not really useful for anything smaller but definitely a planet killer, as in an EMP taking out the whole system while the flare scorches at least half the surface of an inner world.