Would a 24mm/40mm pancake or 50mm prime be better for landscape photography with a Canon APS-C camera?
The EF-S 24mm f/2.8 STM, the EF 40mm f/2.8 STM, and several varieties of EF 50mm lenses meet all three of your criteria when compared to your current EF-S 10-18mm f/4.5-5.6 IS STM and whatever variety of the EF-S 18-55mm f/3.5-5.6 you own. They are also relatively inexpensive.
Included in the list above would be both the EF 50mm f/1.8 II and the EF 50mm f/1.8 STM. The EF 50mm f/1.4 is about the same size/weight as your 18-55mm kit lens. The EF 50mm f/1.2 L is larger and heavier as well as considerably more expensive.
Regarding the EF-S 24mm f/2.8 STM, EF 40mm f/2.8 STM, and either version of the EF 50mm f/1.8:
- They are all capable of producing sharper images at wider apertures than your current lenses.
- They all have wider maximum apertures than your current lenses.
- They're all smaller and lighter than your current lenses.
- They're all relatively inexpensive compared to many other lenses.
Please realize that the single most significant tool for increasing the crispness of your landscape photos is a stable tripod and a remote cable release. Not only does a tripod and cable release eliminate the blurriness that results from camera movement when taking handheld shots, but it allows for longer exposure times that in turn allow for narrower apertures where almost all lenses are sharper than when used wide open.
With most zoom lenses some focal lengths will be a little sharper than others. With some zoom lenses some focal lengths can be a lot sharper than others, particularly telephoto zooms with a very large focal length range.
Saving your images as raw files and optimizing micro-contrast via the unsharp mask and other tools in post processing can also make your images much crisper than a straight out of camera jpeg of the same exposure.
Before you spend money on a new lens make sure you are getting the most out of your current lenses by using good stabilization techniques, shooting at your lens' sweet spot in terms of aperture and focal length, and getting the most out of what you have captured in post processing.