For World Partition, there is no difference between using a file tile set or a single PNG file heightmap, the tiles will be stitched during the import process, so you gain nothing.
And using tiles doesn’t allow you to create a larger terrain size.
UE5.3 is using way less memory during the Landscape import process.
So depending on your system specs you should be able to import much larger heightmaps in World Partition now with 5.3.
World Partition importing with Edit Layers enabled is limited to 16k x 16k, with Edit Layers disabled is limited to 32k x 32k.
If you want to go larger than that, you can still use World Composition.
UE4.x is only using World Composition, so it supports much larger size terrains.
I have imported heightmaps up to 260km x 260km in World Composition in both UE4 and UE5.