That gets me to thinking... Warp drive avoids relativistic concerns by creating a subspace bubble so you aren't going fast you're sort of warping space around you. If impulse gets you up to 0.5C as happened in one TOS episode, then the universe would be going 15% faster than you, and it increases from there. I'd want to limit solar system travel in that case!

Impulse engines are probably too large and expensive to put on something like that, and you might need big starship-grade ones to be able to get something moving that quickly.

You would also want a tough deflector system, so that it isn't immediately obliterated by the first piece of space dust that it runs into, and a nice computer so that it doesn't just fly off into the first star 'til morning, and at that point, it might just be too expensive to bother with. You're basically building a small starship at that point.

One argument I tend to see when this comes up is, that the shields would block it. Which then makes you think, if they could block that, then what couldn’t they block? It makes them pretty much invincible. So I don’t think that’s it.

At least in Trek, 24th century shields are extremely good at handling physical impacts. Even in a compromised state, with one of the shield processors having escaped and gone rogue, an ageing California class was able to take several hard impacts without so much as a scratch. They might have been able to resist indefinitely if their shields were performing optimally.

Earlier ones, like the ones fitted to the Kelvin-era Enterprise, are much weaker against physical impacts, such that the wreckage of a starship was able to still cause major damage to the outer hull, even with shields at full.

Shields are probably much weaker against energy fire, since they appear to be an outcropping of deflector screen technology, and as such, fare better against physical objects. A powerful energy weapon probably does a much better job at knocking down shields than just smacking them with the physical photon torpedo, even if the torpedo is fired at warp speeds.

My head canon at least is that impulse engines don't work alone to drive a 700 kilotonne ship to 0.25c. The deflector uses subspace to effectively lower the mass of the ship to something capable of being driven to those spreads. Otherwise Einstein comes and fucks with you.

Honestly I think the answer to “Why don’t we see weapons like this in Star Trek” is that they just have cooler weapons. I have to imagine that the energy weapons the average ship is packing could do similar, if not much more, damage to a planet, but could also do much less more precisely.

Why don't they use nukes? Or simple orbital impact? Probably considered a waste of a good planet or forbidden in treaties or something, but when the Dominion massacred Cardassia you'd think they wouldn't give a toss about that sort of thing.

Why not do like Hemmer and just beam out a chunk of the planet's core? Done strategically, this could have a similar impact to the bomb planted on Qo'nos in Discovery.

Why waste a whole Voyager when you can rig two quantum torpedoes to make the entire planet's atmosphere uninhabitable?

In the Star Trek universe, if you're intent on "glassing" a planet, it's in one of two scenarios:

1. The planet inhabitants can't fight off a single star ship, in which case you could just park in orbit and bombard to your heart's content, with the option of either precision strikes or complete annihilation, without expending anything other than the energy it takes to power the ship.
2. The inhabitants can fight off a star ship, in which case they likely have the technology to detect such a weapon at sufficient range to intercept/destroy/redirect it, or planetary shielding powerful enough to stop it.

In the latter case, you could put the effort into adding a cloaking device to the weapon to get around that. But in that case, why not just use a regular cloaked ship to delivery some other payload? There are tons of examples in TNG of narrowly-averted planet-killing disasters only prevented by careful engineering. Probably way easier to actually cause the disaster. Examples include igniting the atmosphere, causing geographic instability/earthquakes/volcanic reactions, exploding the system's star, crashing a natural moon into the planet, unleashing a biological weapon...