There should be no issues with importing a 16321x16321 heightmap into a World Partition level if the computer has at least 64GB of memory.
Heightmap importing doesn’t seem to like to use larger Page File setups.
You could use World Composition, which allows for subsequent tile importing, but that has been deprecated in UE5.x.
World Partition can also be extended to larger sizes in 5.0 to 5.2 by using the tile brush import system (I can provide a YT video link if you are interested).
For UE5.3 World Partition subsequent heightmap importing, I plan on testing that feature shortly to see how it works.
UE5.3 Preview seems to only have a new Landscape Cell “Region” for larger selection of a region to load and unload.
There doesn’t seem to be any new “subsequent import” feature yet that I can find.