One thing you need to factor in is isostacy. The continents of Earth and the crust of planets aren't a simple layer spread over the outside. They are typically made of different, lighter, stuff and they "float" on the denser material underneath.

(Remember that all these material are as strong as mild steel, so things don't bob in the waves, they drift serenely on a geological timescale)

On Earth, the continents are typically 35 km thick, and they float with a freeboard of about 3.5-4 km (~ 10%). In Mountainous areas, there isn't just a heap built on top of the crust, but the entire crust is thickened, to as much as 60-70 km. Here the freeboard is up to 6 km, so we have mountain ranges, and individual peaks can be taller, supported by local strength of the crust.

On the planets, much the same applies. The crustal dichotomy between the southern highlands on Mars and the northern plains is a ~4 km elevation difference, caused by a tripling of crustal thickness. Basins such as Hellas, Argyre, and Isidis are local holes where much of the crust has been removed in the original impact. Some probably reformed since, and the sediments help, but you can't simply "fill up" the hole.

If you try, just like a sedimentary basin on Earth, the rocks subside and you are left with 90% of the hole you used to have, so you have to do it again, and again, and again.

On a planet with limited erosion and transport, there hasn't been enough time to fill things like Hellas.

Even the buried basins of the northern plains won't stay "buried". They kept on subsiding, even after they were smoothed over, so we see large depressions above them.

On the Moon, the Highlands/Maria distinction is similar.

On planets that don't have a chemically distinct crust, isostacy can obliterate things. For instance, on Europa there is a shell of ice over water or slush. An impact basin there rebounds, and the ice at the base of the crust freezes into new crust, which rebounds further, and more freezing occurs until isostacy has removed the hole entirely.

There is a complication on Mars related to timing of impact and secular cooling. As well as the crust, which was established early in its history (pre- 4.0 Ga), there is a thick skin formed by cooling of the uppermost parts of the Mantle. This Lithosphere includes the crust, but is much thicker (300-400 km today). It is quite strong, and so can support things like Olympus Mons with ease, although there is a depression around all loads on about a 1000km lengthscale.

Everything balances, but the way in which it balances may vary.